You can use the "Log Out" button to sign out of an account you are currently playing on and swap to another, such as your Guest account or Facebook account. Go to your Settings menu (Mailbox > Settings tab) and on the right side of this window, tap the red "Log Out" button. This will log you out of the account you were playing on currently and bring you to the title screen. On the title screen, you can choose to sign in with any other account.

Note that once you sign into an account for the first time in your game, the credentials are saved in the title screen and cannot be removed. For example, if you sign into a Google account, your credentials for this account will remain saved and you will not be able to sign in to an alternate Google account.

There is a workaround for this, however. If you wish to use an alternate account rather than the one saved in your game, you can clear the app cache and/or uninstall the game. This will effectively remove all local data and allow you to log in with a different account.

Note: Do NOT clear the app cache or uninstall the game if you are on a Guest account only, as the data will be lost.