Windows: * Windows XP: %SystemDrive%/Documents and Settings/%USERNAME%/Local/Application Data/UNDERTALE * Windows.
If you're not playing the Steam version of the game, the simplest method to remove the consequences of completing a genocide run is to fully uninstall and reinstall the game.
When you next start up the game, it will be as if you had never done a genocide run ever before.
Right click the file system_information_962 or system_information_963, depending on if you are on post-genocide black screen, or post-genocide post-soul-sold.
(Default on Windows: \AppData\Local\UNDERTALE) While it's possible to clear these files out, it's complicated, and a simpler solution exists on Windows if you have sufficient access rights on your computer. Steam Cloud will automatically restore those files, and it also keeps a local cache of the files to restore them even if you're offline. If you're running the Steam version of the game, deleting files in the save directory won't be enough.