There is a square bolt on the right hand side towards the front of the transmission secured by a locknut (19MM I believe). Undo the locknut, turn the bolt, 1/4 turn clockwise and test. Don't tighten more than a quarter turn at a time or the trans will attempt to go into reverse between shifts. Not a fun thing.

The odds are if it isn't going into reverse the trans is worn out. Give it a try. You could get lucky.