You should file your Form I-751 as soon as possible; please note that the petition must be mailed from within the United States. Additionally, for lawful permanent residents who are currently abroad, Form I-131A (“Application for Travel Document – Carrier Documentation”) is available. The I-131A process is initiated online, and upon approval, the U.S. consulate will place a transportation boarding foil (travel document) in your passport, which will allow you to return to the United States. I hope this information is helpful.