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What version of XOOPS are you using?
Templates are stored in the database and that's where you need to edit them (or, when using 2.0.14 or 2.0.15, override them by placing them in your themes/yourthemename/ folder -read the 2.0.14 release news and pages for info on how to do that). If you want to edit a template in the database, go to system admin -> templates, clone the default template set (you can't edit the default templates), and edit the template in the cloned set. Save, and that should show your changes.
Herko