File-> Open .osu in notepad
Open beatmap folder, redrag audio [probably named audio.mp3] into osu (may have to rename the audio if the same map pops up).
Create a new difficulty in the new map you made (don't bother naming and stuff, it will be rewritten later).
File-> Open .osu in notepad again and copy paste your map into into the .osu file
Change these values to this v
Creator:[yourname]
BeatmapID:0
BeatmapSetID:-1
Change
Tags:
to correspond to your map.
Save.
Remember to properly credit any assets you used (audio, timing, background, hs, etc.).
Good luck!