If you have no MOD's I suggest this method:
*NOTE: Before proceeding backup all files and the database.*
- Upload all new files from a download of the full version, EXCEPT for config.php, the /images and /files folders.
- Go to yoursite.com/forumfolder/install/database_update.php
- Let that run and then delete the install folder.
- You now have a new up to date board containing all your users and posts but without any MODs that you might have had installed.
Please note any 3rd party styles will also require updating.