<html><head><script src="https://libs.baidu.com/jquery/2.1.4/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>