how to add and customize items in menu/nav bar in header

1) To remove sign up and sign in links edit your default.xml code at below location.

/app/design/frontend/Infortis/base/Magento_Theme/layout/default.xml

Add these lines of code

<referenceBlock name="top.links">

    <referenceBlock name="register-link" remove="true" />           <!--for sign up Link-->
    <referenceBlock name="authorization-link" remove="true" />      <!--for Sign In Link  -->

</referenceBlock>

2) Add more links with account links enable this block Custom Top Links (to replace Magento's default Top Links)

Go to Admin > Content > Blocks >  Custom Top Links (to replace Magento's default Top Links) > Enable

Place your custom links in this block

3) Add custom links in main menu with Home these links come at right side by defaul

Go to Blocks and edit this block "Main menu - custom links" add your custom links for main menu here.

see below picture , I put some custom links at both place and removed the links which you wanted to remove

4) Placing custom links next to Home Link

For this you can use this simple method of placing custom links:

• Create a block and place your content in this block ,

  • Note: This is block you need to assign in category

• Create a sub category to root category

  • i.e Name it My Test Link
  • Set "yes" to include in menu and enable category
  • In content section > Add CMS Block > Select your block
  • Display Setting > Display mode > Static Blocks only
  • Save the category and then Drag it to set the order, Like I dragged it to first position to show just after home link

All done !

Keep cache disabled / flush your cache

I use the Ultimo Theme on a Magento 1.9 instance, but the method is the same for 2.1.

In the user guide, there is a section (starting on page 80) that covers built-in blocks. In the sub-section 6.2.3 List of built-in static blocks and containers, you will find that the block-identifier you are looking for is block_nav_links, and if you want to add content in a dropdown, it is block_nav_dropdown.

There is a link in that section to 12.3.1 Custom links inside a static block that shows an example of what the html should look like inside your block with an identifier of block_nav_link:

<li class="nav-item level0 level-top right">
    <a class="level-top" href="{{store direct_url='about-magento-demo-store'}}">
        <span>Sample Internal Link</span>
    </a>
</li>
<li class="nav-item level0 level-top right">
    <a class="level-top" href="http://infortis-themes.com/">
        <span>Sample External Link</span>
    </a>
</li>