How to fix a broken/stuck speedometer?

Something probably went wrong with the jump start. I would probably attempt to take it apart and see if you can get the needle to go in the right place, but it will be one of those jobs where you have to be real careful to not break anything. Worst case you will either have to take it to a dealer to get the instrument cluster replaced and the odometer set or get one from a junkyard and get the odometer set.