Hi Paul,
If you have no MODs installed
Step 1
Download the “Changed Files Only” from the download page at phpBB.com, and unzip it to a directory at your hard drive
Step 2
Inside of this zip you'll see several more zip files. Unzip the one that refers to the version you're updating
Step 3
Upload all those files, while overwriting all old ones. You should also upload the /install and /contrib directory which can be found in the main zip file (phpBB-2.0.xx-files.zip)
Step 4
Run /install/update_to_latest.php trough your browser, using
http://www.domain.com/phpBB/install/..._to_latest.php, for example
Step 5
Delete the /contrib and /install directories from your server
Source:
http://www.phpbb.com/kb/article.php?article_id=271