The following is copies from the USCIS website:
http://www.uscis.gov/portal/site/uscis/menuitem.eb1d4c2a3e5b9ac89243c6a7543f6d1a/?vgnextoid=820a0a5659083210VgnVCM100000082ca60aRCRD&vgnextchannel=820a0a5659083210VgnVCM100000082ca60aRCRD
If Your EAD has Expired
If you are still eligible for work authorization but your EAD has expired, you should file for a renewal EAD by submitting a Form I-765, Application for Employment Authorization. You cannot file for a renewal EAD more than 120 days before your original EAD expires.