If you're already using phpBB 2.0.18 or lower, you can easily upgrade your board by following these instructions:
Make a full backup of all files and database (if you don't do a backup, don't dare to ask for support! )
Did you backup everything? If you didn't make the backup, please, step back to the previous point.
Copy your current config.php and keep it in a safe place.
Check again that your backup is ok and keep it in a safe place.
Login as administrator.
Upload and launch install/update_to_xs2.php (i.e.
http://www.mysite.com/forum/install/update_to_xs2.php).
Upload all phpBB XS 2 files in the same folder where phpBB is installed and running (all files but config.php). Pay attention that all older files must be replaced by new ones.
Apply all permissions listed in Fresh Installation.
Delete or rename install folder.
Use your New Forum .