How do I stop chipd support payemnts if the child is 20 years old and not in school?

Are they being garnished?

You need to contact the court clerk and see what you need to do

What state are you in and are the payments through an agency ?

If your payments are being automatically taken from your pay, then you need to go to the agency that is processing them and ask what you need to do to sotop them. It may require going back to Family Court to get an order to serve on the agency.