How to upgrade phpBB3 through SSH
Before you start the upgrade make a backup of the phpBB3 files on your account and of the database used by the application.
First, go to the download section of the official phpBB3 site and download the patch files archive for upgrading phpBB3. The archive contains several patch files and two folders. Upload to the root phpBB3 directory on your hosting account only the patch file that corresponds to your current phpBB3 version (e.g. phpBB-3.0.8_to_3.0.9.patch) and the install folder.
Then access your account through SSH and execute the following command:
patch -cl -d (path to phpbb3 directory) -p1 < (path to patch file)
Type the actual paths to the root phpBB3 directory and to the patch file in the place of (path to phpbb3 directory) and (path to patch file). There are no brackets around the actual paths in the command.
The update process is almost instantaneous. If there no errors, you'll see only a list of the updated files.
Then you have to open with your internet browser the URL address of the database_update.php file. It is inside the install directory that you uploaded earlier to the root phpBB3 directory on your account (e.g. yourdomain.com/phpbb3/install/database_update.php).
The last thing you have to do is to delete the install directory and the patch file from the root phpBB3 directory on your account.
You can also check out the phpBB3 SSH Upgrade Tutorial.