I'm currently paying child support for a child that's not mine and have the DNA test to prove it. How do I stop payment?

If you have a DNA test that shows you are not the father of the child in question, you can file a "petition to disestablish paternity".  Since you are paying child support I assume that you have a court order that did establish you as the father and ordered you to pay support.  You apparently have evidence that shows you are not the father and that would be the basis for the legal action  called petition to disestablish paternity.  This is a pleading that must be filed with the court and served upon the other party (individual petitioner) and copy to the department of revenue (I assume that the department of revenue was involved in the original action).