You can file a "Motion to modify child support order," with the same court that ordered you to pay. But, if you are wanting to change the order because you are not the father, you should go and have the DNA test taken. If they won't allow you to have the DNA test taken, you could file a "Motion to modify child support order and for Paternity test," and ask that the court order the child to undergo DNA testiny and that you go and have yours done.
You may need to retain an attorney in your area to help you with this matter. You can call your local (city or state) Bar Association and ask for a referral. If you cannot afford an attorney you can do one of two things (1) seek free legal aid by calling the Bar Association and asking for the phone number(s) (2) ask the clerk of Court for a form to file the Motions to modify child support and for Paternity testing and the clerk should help you. Tell the clerk that you do not have an attorney.