Changed alternator to correct problems, now everything runs fine but the light won't go off. How do I reset the light?

 

Light should go off on it's own, indicating the alternator is putting out a higher charge than the battery drain...are you sure the new alternator is putting out a charge? Was it a new or rebuilt unit? Sounds like the internal voltage regulator is not working, assuming the alternators in the new Fords have internal regulator units. Installation of alternators these days are "plug and play" so to speak. Not like the old days of generators, where you had to "polerize" them.

If it were me, I'd first double check the wiring connector to the alternator (no broken wires), then remove the alternator and take it to a parts store and have it tested.