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.