What state?
Contact Child Support Enforcement of the state that has jurisdiction.
Each state has it's own procedures....
In AL, I just had to go down to their office and say "Hey, girls emancipated, and it's fine with me to close our case."
They gave me the form to sign, flipped the switch in the computer, and it was done. In this case, I'm the CP.
Other states have to have a motion and stuff....so it's really hard to give you one right answer.