What should I do if there was a mix-up and I realized that I have overpaid child support?

It depends on whether there is a current order or if the order has ended because your child has emancipated. If the child is emancipated but you continued to pay you will need to go back to court and get an order that the mother repay you. If the support is ongoing, go to the child support enforcement agency. They should show an overage on the account and should be able to reimburse you.

You will not get any money back from child support unless the obligation order was closed and there was an overpayment; You should get some type of credit on next month, but I would let them know because they surely won't let you know.

You need to file a motion with the court for modification of child support. You should get an attorney for this.