The easiest way is to change the Publisher ID in the Adsense code from his to yours.
<script type="text/javascript"><!--
google_ad_client = "Put your Publisher ID here";
/* 728x90, created 9/24/08 */
google_ad_slot = "7121941508";
google_ad_width = 728;
google_ad_height = 90;
//-->
</script>
google_ad_client = "Put your Publisher ID here";
/* 728x90, created 9/24/08 */
google_ad_slot = "7121941508";
google_ad_width = 728;
google_ad_height = 90;
<script type="text/javascript">
src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>
You can find your Published ID at the top of your Adsense account.