<div id="div1" class="mavi">
  
</div>
<button id="add"> Class Ekle</button>
<button id="delete"> Class Çıkar</button>
.mavi{
  width:200px;
  height:200px;
  background:blue;
}
.kirmizi{
  width:200px;
  height:200px;
  background:red!important;
}
$(function(){
  $('#add').click(function(){
     $('#div1').addClass('kirmizi');
  });
  $('#delete').click(function(){
     $('#div1').removeClass('kirmizi');
  });
});

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

  1. https://code.jquery.com/jquery-3.1.0.js