Based on the "I know nothing about DNN" my first reaction is to suggest that you hire someone knowledgeable about DNN to do this for you.
There are legacy versions of DotNetNuke on SourceForge that go back as far in time as your installation.
If you want to upgrade, you'll probably want to upgrade first to a 3.x version, then to 4.x, and then to 4.6.2. From there you can jump to the current version. There may be some steps in between, but I do know (and you can tell from the matching timestamps on the versions) that there were a series of 3.x and 4.x versions that differed only in the ASP.NET version required. So, it's easy to go from 3.x to the corresponding 4.x version.
And, assuming that you don't have custom modules to any great degree, someone who knows what they are doing should be able to do this update for you with 2 or 3 days of work -- and I'm kind of wagging a wet finger in the wind to make this estimate. Certainly the size of the site, what modules you are using, available resources, etc. will be important.