The save files are stored in your Steam library's common folder. They are named slot0.rdsave , slot1.rdsave , and slot2.rdsave , corresponding to the three in-game save slots. Save File Location You can find the save files at the following path: SteamLibrary\steamapps\common\Rhythm Doctor\User Updating or Modifying Save Files
The core file you are looking for is actually SaveData.rmd or SaveData.rmd.bak located in your AppData folder. When we talk about an "UPD," we are talking about the of this file. rhythm doctor save file upd