Where are the save game files stored?

WINDOWS

You can find your saved game files in the following folder:

Steam: %USERPROFILE%\Saved Games\Grounded

For the Windows 10 Store or Xbox PC Game Pass versions of the game, players cannot 
access the save files as they are managed by Xbox. There is an option in Continue Game 
that allows players to export their save for the Steam version of the game.

%USERPROFILE% typically defaults to "C:\Users\<your login name>" but you can enter it into Windows Explorer as shown above.