<style>
	.att02 div	{
		border:dotted 1px red;
		margin: 10px; padding: 10px;
	}
	.bac_lightcyan	{background: lightcyan;}
</style>
<script>
	$(function(){
		$('.att02 .eleclass02').attr('class','bac_lightcyan');
	});
</script>
<div class="att02">
	<div class="eleclass01">要素1</div>
	<div class="eleclass02">要素2</div>
</div>