Apparently, Tecmo Bowl is one of the most modded games ever. Much of this information is found on the tecmobowl.org forums, along with more software.
- Tecmo Bowl Team Manager
Tecmo Bowl is the simplest title. Tecmo Super Bowl got a little more complex, so the editor is more complex. Personally, as I was tinkering, I enjoyed replaying the original vs Tecmo Super Bowl, but that's just me. All things aside, you need an editor.
These programs allow you to load the game file (ROM, see below) and edit team names and rosters through a desktop app. If you're on a Mac and want to run Tecmo Bowl Manager, I can send more detailed instructions, because I got that running as well.
- Tecmo Bowl (or Tecmo Super Bowl) ROM
The game files are called ROMs (read-only memory) and you edit these with your custom versions. The manager apps above make the changes in the original code and you can save out new versions with your customization. If you're just changing team names and rosters, that should be pretty straight forward to pull off.
Emulators play the ROMs as if it was the original console. The best thing about these old games is that our computers are light years beyond the original console, so they run great. You can download NES/SNES emulators from all over the place. I'm using Open Emu becuase it runs multiple consoles in one app.