Here the class "demo" is removed using removeclass function.
$(document).ready(function(){
$("#test").removeClass('demo').addClass('replace');//To replace "replace" class with "demo" to <div> tag.
});
<div id='test' class='demo'>Welcome to hiox</div>
<style>
.demo{font-weight:bold;font-size:1.2em;color:red;font-style:italic;}
.replace{font-weight:normal;color:blue;font-style:normal;}
</style>