After backing up my previous installation, I downloaded the latest version of PostNuke. I then followed the steps listed here. (They should do something similar to gallery, where the archive always expands to the same name, so it is easy to do upgrades).
Once that was working, I tried to go into the forums, but this didn't work because the older version of PNphpBB doesn't work with the latest PostNuke. I then downloaded the latest PNphpBB, and expanded it on top of the previous installation.
Everything was working fine, until I noticed that my theme wasn't looking "right". All of the links had the borders around them, while they previously didn't have them. I then tried to activate a new theme, but I got my installation in a state were I couldn't see anything. I had not activated Xanthia, so most of the themes were not working. But since I had selected a theme that depended on Xanthia, I couldn't activate Xanthia.
I download and used the PostNuke Swiss Army Knife to select a simple theme. Once I did that, I was able to activate Xanthia.
The only other problem that I had was that I removed the "install" directory from the PNphpBB directory before I activated Xanthia. I had to expand the archive again so I could upgrade the PNphpBB installation