“How do I lower my child support when I get a lower paying job?”
You bring a motion with the Court and ask them to lower it. Ideally you will have a lawyer do this, because the Courts are notorious for not playing nice with people who represent themselves in family law, but if you cannot afford an attorney, you should ask the clerk at the courthouse how to file a motion to reduce your child support.
Whatever you do, do not rely on an agreement made with your ex but not signed by the Court. Child support is unique in that, in many states, even if you have an agreement in writing, if it is not cemented by an order of the Court, it is invalid. I know many a person who has reduced their child support following an agreement with their ex, only to have the Court tell them months – and even years – later that they owed a massive amount of back support based on the old amount because the agreement had not gone through the Court.