phpBB XS is just phpBB with some MODs added. What you need to do is remove the MODs by going to a fresh install of phpBB.
Do you have the latest version of phpBB XS?
First step and very important: Make a backup of your database and of your forum files.
2) Delete all your present forum files except for config.php
3) Get the full phpBB2 package from the Downloads
4) Upload all the files except for config.php and the install & contrib directories.
5) Get, install and run the Clean phpBB SQL Tables
6) If you do not have the latest version of phpBB XS, you need to update your database to 2.0.21. From the download package, upload install/update_to_latest.php and run it.
You will now a have a clean phpBB forum with no modifications.