You need to edit the XQL file for the database you are using or the generic XQL file. In this case category-browse-subcategory.xql.
When you do that you'll also need to visit acs-admin/apm/ and reload the ecommerce package to update the copy of the query the server is using. A reload link next to the ecommerce listing will be shown if any tcl or xql files have changed since the server was started.
