The first step in upgrading is to obtain the latest release of Active Forums.

Before Installing
As with any upgrade, you should always backup your current installation.  Make sure you back your database and all the files and folders in your DotNetNuke directory.

Next step would be to read the ReadMe.txt to see if there are any issues you need to be aware of prior to upgrading.  Once you are ready to upgrade, install the Private Assembly zip file just like you would any other module.