<script type="text/javascript" src="/js/jquery.js"></script>
<script type="text/javascript">
$(document).ready(function(){
$("button").click(function(){
$("p:first").removeClass("intro").addClass('main');
<h1>This is a heading</h1>
<p class="intro">This is a paragraph.</p>
<p>This is another paragraph.</p>
<button>改变第一个段落的类</button>