Hi Medar! It's been awhile
Basically you're looking at a couple steps. First you'd need to download 6.5.5 from the Members Area. And then upgrade your current version to that. It's fairly simple, just follow the instructions in the UPGRADING.html file.
At that point, you can install UBB.threads 7.2.2 and then download the threads importer zip from the members area, using that you'd import all of your current data into 7.2.2. Steps for both the install and the import can be found on the
Documentation Page.
And we're always here in case you get stuck anywhere along the way.