<html>
<head>
<script src="https://libs.baidu.com/jquery/1.10.2/jquery.min.js">
</script>
<script>
$(document).ready(function(){
$("button").click(function(){
$("p:last").removeClass("intro").addClass("main");
});
});
</script>
<style>
.intro
{
color:red;
}
.main
{
background-color:yellow;
}
</style>
</head>
<body>
<h1>This is a heading</h1>
<p>This is another paragraph.</p>
<p class="intro">This is a paragraph.</p>
<button>Change class name of the last p element</button>
</body>
</html>