You can easily download phpBB from phpBB.com and update your files uploading everything except CONFIG.PHP and then run this replacing with your domain name and forumpath
http://www.domain.com/phpBB/install/..._to_latest.php
Once that runs you delete the install and contrib directories and you should be fine. They have full upgrade instructions on there site though this is just what I remember from doing phpBB upgrades for customers