Family Law Questions? Ask a Family Lawyer Online.
You take a copy of your divorce decree (it should clearly state when child support ends for each child) to your HR department and that should be sufficient. They have to follow the decree. If that's not enough, you can contact the OAG office in your area and ask them to issue an updated withholding order.
If your decree doesn't state what the new amount should be, you'll have to go through the OAG. In Texas, you pay 25% of your take home income for two kids, and 20% for one, so your support doesn't go down by half. The below link is a good place to start.