POD 2.0 is, in fact, still sold and supported. While I now use an HD500 for gigging, my POD 2.0 was my first love (modeling wise anyway), and they'll have to pry it out of my cold dead hands!
Line 6 is really good about keeping manuals online, even for gear that hasn't been sold for years. I think you can find what you are looking for here. Look in the POD section.
You cannot install different models on the Pod 2.0. Perhaps you are referring to tones?
In any case, the manuals are available here:
Advanced User Guide:
Download and install the Monkey program. Run this and it will show you what programs you can download and install (specifically you will need Line 6 Edit).
Line 6 Monkey (I am assuming you are on a PC with Windows XP) is available here:
(If your computer is something other than what I have assumed just go to http://line6.com/software/ and from the drop-down menus select Pod 2.0, Line 6 Monkey and whatever OS you are using).
Line 6 Edit will allow you to download tones from http://line6.com/customtone/browse/pod20/ which you can then transfer into your Pod 2.0 using Edit. To do this you will need a MIDI interface. If you do not have one a cheap option is a USB to MIDI adapter. Line 6 recommend the M-Audio MIDISport Uno as a preferred option as this is known to work.
Other helpful supporting documents: