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.