Use a tool like the Base64 Decode to convert it into readable text.
If you had to decode the file in Step 2, you must re-encode it before the game can read it again. Copy your modified text.
Saves are typically located in the internal storage at Android/data/com.monomyth.grimquest/files/ . Note: Accessing this folder on newer Android versions may require a PC connection or a specialized file manager like ZArchiver . How to Use a Manual "Save Editor"