If the door is unlocked remove the knob and take it to a locksmith for a rekey.
If you want to save a little bit of money check with anybody you know that has teenagers and see if any of them know how to pick locks. It seems a fey years ago all my kids and their friends all got into picking locks. So it's entirely possible the teenagers in your area did the same. If you can get the lock picked have them leave it with the cylinder turned and then take it to the locksmith for the rekey. Let them know that the cylinder is already turned which may save on paying the locksmith to sit and try to pick the lock.