To prevent Steam from overwriting your new save with your old one:
Download a reputable from a trusted community source (like Speedrun.com or r/bindingofisaac). Ensure the file is named correctly (e.g., rep_persistentgamedata1.dat ). binding of isaac repentance 100 save file
Installing a pre-made save file (often found on Speedrun.com or community forums) requires manual file management: To prevent Steam from overwriting your new save