How can I get an extension on my green card after I've filed an I-90?
Make an appointment with your Local USCIS office.
If you need to request an extension after filing Form I-90, USCIS requires that you make an appointment with your Local USCIS office and explain why you need an extension. CITATION: Page 98; http://www.uscis.gov/sites/default/files/USCIS/About%20Us/Electronic%20Reading%20Room/Customer%20Service%20Reference%20Guide/Permanent-Residents4-3.pdf