In UBB7 you'll edit one of the template files.
Admin templates are in templates/default/admin
Normal templates are in templates/default
As for the name of the templates, it's quite easy to figure out which one to edit. Also, all normal templates have the extension .tpl (wheras admin templates are tmpl).
Take a look at the URL on the page you wish to edit, after ubb= (or ubb/) there is a single word, for example here it is "showflat"; so the template for this page is "showflat.tpl" (since its a normal template, and the page name is showflat).
So the file would be templates/default/showflat.tpl
As for UBB.T6, I'm not entirely sure how the naming scheme went on things since I never really used it.
