I am so sorry that this has happened to you.  What you need to do is to bring a proceeding in court for modification of the support order and you need to put forth to the court your proof that you are not the Father of these children.  You could also ask for confirmation by requesting that the court order DNA testing on all of the children, should your proof be less than definitive.  DNA testing is really what the courts like to see, other than maybe an affidavit of paternity.   Go down to the court in which the order was issues (just a note: the proceeding has to be in the same state where the children are located) and ask them where you file the paperwork.  You may want to consider getting an attorney involved.  Good luck.