skin1/customer/main/product.tpl
Button...
Code:
{include file="buttons/button.tpl" style="button" button_title="Back to Product Category" href="home.php?cat=`$current_category.parentid`"}
or hyperlink...
Code:
<a href="home.php?cat={$current_category.parentid}">Back to Product Category</a>
Or you can change 'parentid' to just 'categoryid'.