For Windows:
The Unity save file is in
C:\Users\
<username>\AppData\LocalLow\OppaiComics\FlirtyF
If you haven't used up all your save slots, the safest thing to do is to make a new save before starting this process.
If you use the save editor to load the most recent save file, look for:
The value after that may be blank for default, or may have the value "non" there. change it to
Code:
"PlayerName": "<your name>"
Save/download that changed save file, put it into the safe directory listed above.
Then start the game, load that save and the name should be fixed.
Edit: added missed steps