How to modify child support arrears

That really depends on the judge. You need to request that she pay you child support since the child is with you. What she may owe you could potentially be set off against what you owe.

Generally speaking, child support can only be modified from the date of filing forward. However, there is a statute that might allow it to be modified retroactively if the other parent did not observe the timesharing schedule that is in place. Based upon your question, I am not sure if you had a timesharing schedule in place. I have seen judges retroactively modify support arrears even though it was technically not proper. Consult with a family law attorney ASAP.

I agree that support arrears generally cannot be modified prior to the filing of a petition for modification. It the guidelines were based on substantial timesharing and the co-parent has not exercised the timesharing, a retroactive modification is in order. I would also plead in the alternative if that theory fails, to establish ongoing support, imputing income to the ex and asking the court to offset the ongoing support versus the accrued arrears. Sounds like primary care, visitation, parenting plan and/or timesharing may need to be modified as well.