This is what the game intends for you to be able to do:
1. Run game
2. Go to Mods
3. Click "reset"
4. Run game again, and should be back to stock.
Problem is some versions corrupted the backups folders. While in other situations, if the current files are corrupt, then you can't open the mods dialog, or use the reset button anyways. You can manually delete the files folder and replace it with a copy of the backup folder in the game folder, but the backup folder may be suspect. Thus best bet is to wipe the game folder and extract it again. Or extract it to some new location and use that.
0. Optional: Delete and re-extract mod, to ensure it's not been corrupted.
1. Delete and re-extract game
2. Run game, go to Mods, and apply
3. Run game again, mod should work.
Step 0 is maybe needed if you edited the mod in some broken way.