When first logging into the game, you can either play as a guest or use one of the available services to link your account to. 

To link an existing account to any of the available services, go into the menu on the Town screen, SETTINGS - ACCOUNTS and choose the preferable service. 

ATTENTION. If you accidentally log into the wrong account (and the game starts from the tutorial again), DO NOT try to get your account via the ACCOUNTS menu. Doing this will unlink your old account from the chosen service and link the new one to it (the game will notify you in advance if another account is linked to this authorization service). To get your account back, close the game, clear the game data and make sure to log in using the correct service from the starting screen. Otherwise you might lose your progress.

If you did lose access to your old account and accidentally linked your credentials to a new one, use that account to contact the game support via the menu on the Town screen.