The fretboard should be removed, the gluing surfaces re-planed, then it should be clamped and glued, and finally the fingerboard resurfaced to bring it true. Bring it to a pro if you don't know what you are doing.
*If it is just one small corner that is lifting it could probably just be reglued without removing the board...however I would have concerns about the rest of the joint giving up the ghost soon.