You sue her in small claims court by proving that you overpaid her and that she doesn't (not "she don't) want to refund the money. You show the rent or lease agreement that says you owe x number of dollars a month for rent, and then you show the cancelled copy of your check that proves that you paid $50 (or whatever it was) more than you should have. She then has to prove why she shouldn't refund the money.