Box 1: Print the county in which you are filing your petition. See "Where do I. File?" in the Introduction for more information about what county you.Discover how to navigate the legal process of modifying child custody or support orders in Pennsylvania. File a Custody Modification Motion. To initiate a child custody modification, you must file a petition to modify custody with the Pennsylvania court. Modifications to child support will not happen automatically. To seek the modification of a divorce decree, petitions must be filed with the court where the divorce was finalized. To seek the modification of a divorce decree, petitions must be filed with the court where the divorce was finalized. You must also submit a proposed schedule, state your reasons for requesting the modification, and explain why these changes will benefit your child. Modifications to child support and child custody often require legal guidance or advice from an attorney.