If you wish to change the Facebook linked to the game, that's what you should do:

• Unlink the unwanted account in game settings.
• Open the Facebook app and log out from your Facebook account.
• Log out from your Facebook in all the installed browsers as well (Chrome, Opera, etc.). If you play on several devices, you should perform these steps on each.
• Reinstall Mystery Matters.
• Open the game, tap Play and log into the preferred account from in-game settings.

If you play on several devices, you should perform these steps on each.

PLEASE NOTE:
You can only have one game account per device. An attempt to have several accounts in game by linking it to several different Facebook pages can cause mixing and losing your progess. If you wish to have several game accounts, it's best to keep them on separate devices.