I found some helpful information from USCIS about renewing your greencard:
http://www.uscis.gov/portal/site/uscis/ ... 2ca60aRCRD

"When to Renew a Green Card
You should renew your green card if you are a permanent resident with a Form I-551 valid for 10 years and the card is either expired or will expire within the next 6 months.

Note: If you are a conditional resident and your status is expiring, these instructions do not apply to you. In that case, you must use Form I-751, Petition to Remove the Conditions on Residence, to apply to remove the conditions on your permanent resident status. For more information, see the “Conditional Permanent Residence” link to the left.

How to Apply To Renew a Green Card
If you are a permanent resident whose 10-year Form I-551 has expired or will expire within the next 6 months, you may renew your green card by filing a Form I-90, Application to Replace Permanent Resident Card (see the link to the right).

How to Renew a Green Card If You Are Outside the United States
If you are outside the United States and your green card will expire within 6 months (but you will return within 1 year of your departure from the United States and before the card expires), you should file for your renewal card as soon as you return to the United States.

If you are outside of the United States when the card expires and you have not applied for the renewal card prior to your departure, you should contact the nearest U.S. Consulate, USCIS office, or U.S. port of entry before attempting to file Form I-90 for a renewal green card.

For more information on USCIS office locations, see the “Find a USCIS Office” link to the right."

Best of luck!