If you would like to reset your account, go to the top right menu, click "Settings", "Switch Account", "Reset Account".
Please be aware that if you do this, all your progress and purchased items will be lost and you will start a fresh game with a new DBID.
If your previous account was registered with an email address and password, you can still return to your old account if necessary, though you may need to disable syncing with iCloud, Google Play Games, or other apps that sync with your game account in order to do so.