Javascript update/increment variable value on click

You're only incrementing a local variable that goes away at end of function. You may do this :

      var a = 1;
      function increase(){
            var textBox = document.getElementById("text");
            textBox.value = a;
            a++;
      }    

<input type="text" id="text" value="1"/>

function increase(){
    var textBox = document.getElementById("text");
    textBox.value++;
}