To upgrade Geeklog you will need to manually install the latest version which can be found by clicking the download button on the homepage or the link below.
https://www.geeklog.net/downloads/index.php?cid=8
Copy over your original Geeklog install with these files and run the install file which will then update your database to the latest version of Geeklog.
At this point you can then install the forum plugin.
One of the Geeklog Core Developers.