Bootstrap 4 change caret on clicking dropdown

Here is how I got this working with Bootstrap4-CSS-fontAwesome & No JS/JQuery.

<a class="accordion-toggle collapsed" data-toggle="collapse" href="#anyId">
  click
</a>

<div id="anyId" class="collapse">
    Hi
</div>

<!-- CDNs for font-awesome, bootstrap, JQuery -->
<link href="//netdna.bootstrapcdn.com/font-awesome/3.2.1/css/font-awesome.css" rel="stylesheet">

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/css/bootstrap.min.css" rel="stylesheet">

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>

<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/js/bootstrap.min.js"></script>

.css

.accordion-toggle:after {
  font-family: 'FontAwesome';/* essential to enable caret symbol*/
  content: "\f0d7";/* adjust as needed, taken from font-awesome.css */
  color: grey;
}
.accordion-toggle.collapsed:after {
  /* symbol for "collapsed" panels */
  content: "\f0da";  /* adjust as needed, taken from font-awesome.css */
}

Thank you @Satheesh Kumar

Here's how I got mine working using only CSS for bootstrap 4.2 and fontawesome. I've used the aria label and added transitions for the icon, using a span class for the icon

CSS:

#accordionExample .btn[aria-expanded=false] span:before {
  font-family: "Font Awesome 5 Free";
  font-weight: 900;
  content: "\f107";
  float: right;
 transition: all .5s;
}

#accordionExample .btn[aria-expanded=true] span:before {
  font-family: "Font Awesome 5 Free";
  font-weight: 900;
  content: "\f107";
  float: right;
  transition: all .5s;
  -webkit-transform: rotate(180deg);
    -moz-transform: rotate(180deg);
    transform: rotate(180deg);
}

HTML:

<div class="container my-5 mw-600">
  <div class="accordion" id="accordionExample">
  <div class="card">
    <div class="card-header" id="headingOne">

        <a class="btn btn-link d-flex" data-toggle="collapse" data-target="#collapseOne" aria-expanded="false">
          Collapsible Group Item #1
          <span class="ml-auto"></span></a>

    </div>

    <div id="collapseOne" class="collapse" aria-labelledby="headingOne" data-parent="#accordionExample">
      <div class="card-body">
        Anim pariatur cliche reprehenderit, enim eiusmod high life accusamus terry richardson ad squid. 3 wolf moon officia aute, non cupidatat skateboard dolor brunch. Food truck quinoa nesciunt laborum eiusmod. Brunch 3 wolf moon tempor, sunt aliqua put a bird on it squid single-origin coffee nulla assumenda shoreditch et. Nihil anim keffiyeh helvetica, craft beer labore wes anderson cred nesciunt sapiente ea proident. Ad vegan excepteur butcher vice lomo. Leggings occaecat craft beer farm-to-table, raw denim aesthetic synth nesciunt you probably haven't heard of them accusamus labore sustainable VHS.
      </div>
    </div>
  </div>
  <div class="card">
    <div class="card-header" id="headingTwo">

        <a class=" btn btn-link collapsed d-flex align-items-center" data-toggle="collapse" data-target="#collapseTwo" aria-expanded="false">
          Collapsible Group Item #2
          <span class="ml-auto"></span>
        </a>

    </div>
    <div id="collapseTwo" class="collapse" aria-labelledby="headingTwo" data-parent="#accordionExample">
      <div class="card-body">
        Anim pariatur cliche reprehenderit, enim eiusmod high life accusamus terry richardson ad squid. 3 wolf moon officia aute, non cupidatat skateboard dolor brunch. Food truck quinoa nesciunt laborum eiusmod. Brunch 3 wolf moon tempor, sunt aliqua put a bird on it squid single-origin coffee nulla assumenda shoreditch et. Nihil anim keffiyeh helvetica, craft beer labore wes anderson cred nesciunt sapiente ea proident. Ad vegan excepteur butcher vice lomo. Leggings occaecat craft beer farm-to-table, raw denim aesthetic synth nesciunt you probably haven't heard of them accusamus labore sustainable VHS.
      </div>
    </div>
  </div>
  <div class="card">
    <div class="card-header" id="headingThree">

        <a class="btn btn-link collapsed d-flex" data-toggle="collapse" data-target="#collapseThree" aria-expanded="false">
          Collapsible Group Item #3<span class="ml-auto"></span>
        </a>

    </div>
    <div id="collapseThree" class="collapse" aria-labelledby="headingThree" data-parent="#accordionExample">
      <div class="card-body">
        Anim pariatur cliche reprehenderit, enim eiusmod high life accusamus terry richardson ad squid. 3 wolf moon officia aute, non cupidatat skateboard dolor brunch. Food truck quinoa nesciunt laborum eiusmod. Brunch 3 wolf moon tempor, sunt aliqua put a bird on it squid single-origin coffee nulla assumenda shoreditch et. Nihil anim keffiyeh helvetica, craft beer labore wes anderson cred nesciunt sapiente ea proident. Ad vegan excepteur butcher vice lomo. Leggings occaecat craft beer farm-to-table, raw denim aesthetic synth nesciunt you probably haven't heard of them accusamus labore sustainable VHS.
      </div>
    </div>
  </div>
</div>
</div>

Demo: https://codepen.io/anon/pen/XoLgbY

In Bootstrap V4 they have dropped the Glyphicons. Below code will work well as per your need. For drop-down icon style, you can use font awesome.

[data-toggle="collapse"]:after {
    display: inline-block;
    font: normal normal normal 14px/1 FontAwesome;
    font-size: inherit;
    text-rendering: auto;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
  content: "\f054";
  transform: rotate(90deg) ;
  transition: all linear 0.25s;
  }   
[data-toggle="collapse"].collapsed:after {
  transform: rotate(0deg) ;
}

<link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet"/>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.0.0-beta/js/bootstrap.min.js"></script>
<link href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.0.0-beta/css/bootstrap.min.css" rel="stylesheet"/>

<div id="accordion" role="tablist">
  <div class="card">
    <div class="card-header" role="tab" id="headingOne">
      <h5 class="mb-0">
        <a data-toggle="collapse" href="#collapseOne" aria-expanded="true" aria-controls="collapseOne">
          Collapsible Group Item #1
        </a>
      </h5>
    </div>

    <div id="collapseOne" class="collapse show" role="tabpanel" aria-labelledby="headingOne" data-parent="#accordion">
      <div class="card-body">
        Anim pariatur cliche reprehenderit, enim eiusmod high life accusamus terry richardson ad squid. 3 wolf moon officia aute, non cupidatat skateboard dolor brunch. Food truck quinoa nesciunt laborum eiusmod. Brunch 3 wolf moon tempor, sunt aliqua put a bird on it squid single-origin coffee nulla assumenda shoreditch et. Nihil anim keffiyeh helvetica, craft beer labore wes anderson cred nesciunt sapiente ea proident. Ad vegan excepteur butcher vice lomo. Leggings occaecat craft beer farm-to-table, raw denim aesthetic synth nesciunt you probably haven't heard of them accusamus labore sustainable VHS.
      </div>
    </div>
  </div>
  <div class="card">
    <div class="card-header" role="tab" id="headingTwo">
      <h5 class="mb-0">
        <a class="collapsed" data-toggle="collapse" href="#collapseTwo" aria-expanded="false" aria-controls="collapseTwo">
          Collapsible Group Item #2
        </a>
      </h5>
    </div>
    <div id="collapseTwo" class="collapse" role="tabpanel" aria-labelledby="headingTwo" data-parent="#accordion">
      <div class="card-body">
        Anim pariatur cliche reprehenderit, enim eiusmod high life accusamus terry richardson ad squid. 3 wolf moon officia aute, non cupidatat skateboard dolor brunch. Food truck quinoa nesciunt laborum eiusmod. Brunch 3 wolf moon tempor, sunt aliqua put a bird on it squid single-origin coffee nulla assumenda shoreditch et. Nihil anim keffiyeh helvetica, craft beer labore wes anderson cred nesciunt sapiente ea proident. Ad vegan excepteur butcher vice lomo. Leggings occaecat craft beer farm-to-table, raw denim aesthetic synth nesciunt you probably haven't heard of them accusamus labore sustainable VHS.
      </div>
    </div>
  </div>
  <div class="card">
    <div class="card-header" role="tab" id="headingThree">
      <h5 class="mb-0">
        <a class="collapsed" data-toggle="collapse" href="#collapseThree" aria-expanded="false" aria-controls="collapseThree">
          Collapsible Group Item #3
        </a>
      </h5>
    </div>
    <div id="collapseThree" class="collapse" role="tabpanel" aria-labelledby="headingThree" data-parent="#accordion">
      <div class="card-body">
        Anim pariatur cliche reprehenderit, enim eiusmod high life accusamus terry richardson ad squid. 3 wolf moon officia aute, non cupidatat skateboard dolor brunch. Food truck quinoa nesciunt laborum eiusmod. Brunch 3 wolf moon tempor, sunt aliqua put a bird on it squid single-origin coffee nulla assumenda shoreditch et. Nihil anim keffiyeh helvetica, craft beer labore wes anderson cred nesciunt sapiente ea proident. Ad vegan excepteur butcher vice lomo. Leggings occaecat craft beer farm-to-table, raw denim aesthetic synth nesciunt you probably haven't heard of them accusamus labore sustainable VHS.
      </div>
    </div>
  </div>
</div>

Here is the codepen demo link

Note: In your demo link they have used Bootstrap V3.