Solved!
From Facebook (browser):
- Down arrow (upper right)
- Settings & Privacy (upper right)
- Settings (upper right)
- Apps and Websites (left side)
- That's where you can remove FoE login with the [Remove] button.
Next, go to FoE app (at least on iPhone):
- Back (upper left)
- Clear the username/password
- Re-enter your username/password
Note: I didn't start the FoE account with a Facebook login. No idea what happens if you do this when Facebook was what started your account.