How do I get back overpayment of Child Support?

I pay $600 a month in Child Support for 1 child and 3 years ago the Judge changed it to $300 per month but $600 is still being garnished out of my paycheck each month. How do I go about getting my overpaid monies back? Do i need to get a transcript of the day in court where the judge changed the support? Anyone have any idea? This took place when I was in Texas, I now live in Washington, Im in the Airforce.