Steam -> Library -> Deep Rock Galactic (right click) -> Properties -> Installed Files -> Browse...

then go to:

Deep Rock Galactic\FSD\Saved\SaveGames

Place "00000000000000000_Player_Slot_S1-5_100%.sav" in SaveGames folder.

Copy your steam ID from an existing save, and paste into new save file (replacing the zeroes).

Load game, ESC -> Options -> Save Menu, and load new save called S1-5_100%.