How to Stop Child Support

My question involves child support in the State of WA
I need to stop my child support payment as my two daughters, younger of them turned 18 this Feb and she will graduate from HS on June 10th. I live in Texas, and divorce decree was done in NM. My ex. lives in WA. She has remarried and has changed her last name. The withholding agency is based in WA, where my ex resides. How do I proceed.
1) Do I tell my employer to stop with holding from my pay. Send company my divorce decree with my daughters birth certifcates etc.
2) File a motion in NM courts: I called NM courts and they sent me paperwork to file a motion.
3) Do both

Thanks for your help,

Your employer cannot legal stop making CS payments until they have a court order stating so. Your word is not enough.

You must file a motion in whatever court made the order.