If the game uses cloud saves, turn off Steam Cloud syncing so the game doesn't immediately overwrite your modified local file with the old cloud version.
If you edited the file and it now crashes, the save file is corrupted. es3 save editor work
Using ES3 Save Editor is relatively straightforward. Here is a step-by-step guide to getting started: If the game uses cloud saves, turn off
If you are a player or developer looking to edit an existing file outside of Unity: Text Editors If the game uses cloud saves