Depends on the airplane. If its just a trainer or a general sport flying plane what 2 slow said is perfect. If you want the hinges to be prefectly centered again you can cut out the spot where the hinge was and replace the balsa. That is a pain in the butt though and probably not worth the trouble.

Sometimes you get lucky and there is just the right amount of space beside the original hinge location to put the new hinges. I have tried all 3 ways but prefer the latter if possible. I just dont want my hinge lines to be offcenter high or low.