You would use the form for a petition for modification of support: http://www.nycourts.gov/forms/familycourt/childsupport.shtml. For a full assessment, schedule a consultation with a Bronx Child Support attorney.
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What does the child support order say as to how long you need to pay?
The above is not intended to be legal advice, but may be used for general information. Please contact an attorney for specific help tailored to your needs. www.figgardenlaw.com
New York does not use forms. You just file a petition using a template available online and serve it on the adversary. A support magistrate should terminate support and adjust payments in favor of the younger child. Realize that the adversary can and will file an upward modification leaving your payments the same.