(moved from Bug Reports because this is not a bug in the software)
If you made no observations/IDs/comments on one of the accounts, I would just delete it. You can then change the Account Settings on the remaining account, editing the email address for it and linking it to Facebook.
If you do need them merged, iNat staff can merge accounts. Please email help@inaturalist.org with both usernames, both email addresses, and the username which you want to keep. Also, please send us email from both email accounts so that we know you control them.