If you have not linked your current account to Facebook yet, go to the game settings and press the "Facebook is not connected" button.
Your account is now linked to Facebook.
If you have two accounts linked to Facebook accounts and you want to change one account to another, go to the game settings, press the "Facebook connected" button and log out of your Facebook account.
The account linked to your Google Play Games (Android), Game Center (iOS), or Device ID account will load.
Go to the game settings again.
Press the "Facebook is not connected" button and log in to your second Facebook account.