Under CA child support laws, what is the procedure for getting child support payments reduced

child support payments: I have been issued a child support payment order that is to high. I have cancelled checks that were payments to the other party for child support that have not been credited towards the past due amount. How do I go about getting the monthly payments reduced since I make no where near what I used to make in a month. I used to have a law firm representing me on these issues, but they have been absolutely no help.