How do I move a jaw joint back if it gets stuck?

Move it back. A dislocated jaw joint is almost always anterior-ally. That means the jaw has been dislocated out of the fossa to the front. If you move the jaw back and up it should go into the normal position in the base of the skull. Dislocation of the jaw joint happens in dental and throat surgery when the surgeon has to open the jaw widely and the jaw pops out anterior ally. As a last resort see dentist.
See oral surgeon. Usually you need help initially to be shown what to do. I'd start with an oral surgery evaluation.