Download the full version from here
http://www.phpbb.com/downloads/olympus.php?from=submenu
Unzip to your desktop.
From the downloaded files/folders delete the
config.php
file and the
files
&
images
folders.
Using a good FTP client like
FileZilla upload the remaining files/folders overwriting the files on your webserver.
In your browser go to
http://yourboard/install/database_update.php and let it run to completion.
Delete the install folder.
Done.
Please note: Any 3rd party styles will also require updating, style and board versions must match