Can I overwrite a current game?

You cannot remove the game from the state / servers yourself. You can start a new game on your device which will remove the settings for your current game and start from the beginning. If your old game is not bound then it will no longer be accessible to you and the base will eventually be removed from the state after some time (at least 2 weeks I believe).

If the game is bound to an external account you can then go back and log into it again if you need to or if you then bind your new game to the same external account that should remove access to the original game.

You can start a new game on your device by going into the Account Option in the Settings menu. Bind the old game first if you want to keep it. Or you can uninstall and reinstall the game app.