You don't have to file a motion unless public assistance was involved. You can come to an agreement, prepare a stipulation and order, and file it with the court. If DCSS was involved, contact them.
If DCSS is involved, they would have to be a party to any agreement, unless they are dismissed. However, if any of the arrears are for welfare reimbursement, it is not so easy to have those waived. If it is just straight child support arrears, if the custodial parent wishes to waive them, then a stipulation will need to be prepared, and signed by all parties, including DCSS, if they were involved.