1. Make a second character.
2. Get the second character to level 6.
3. Go to single-player by switching to Micheal, Franklin, or Trevor.
4. Switch back to the level 6 character.
5. The game should ask you if you want to re-edit your character, but you don't press anything. Instead, you disconnect. for PS4, you can go into the Network Settings and disconnect by unchecking the "Connect to the internet".
6. Go back in-game and it will kick you back into single-player. Then, reconnect your internet, for PS4, I just re-check "Connect to internet".
7. Now, go back to the game and go to Online in the pause menu and choose to select character.
8. It will show you both of your characters, just pick the one you originally wanted to re-edit.
9. The screen will pop up asking if you want to re-edit this character, and you are going to press yes, which will bring you back to the edit screen. Enjoy!
If you ever want to re-edit them again, repeat from step 4, and instantly edit your character over and over again!