To start the game from the beginning, you need to sign in using a Google account that is not currently linked to your game progress.
This can be either:
a newly created Google account, or
an existing Google account that has never been used with this game.
Follow these steps:
Create a new Google account or choose an existing one that is not linked to the game.
Open the Google Play Games app on your device.
Tap your profile icon and select the Google account you want to use for the game.
Make sure this account is set as the default account for the game.
Launch the game.
If the game is already running, use Log Out / Log In in the game settings (if available) to force account synchronization.
When the game finishes loading and syncs with the new account, it will start from the beginning.