While a green card is proof of permanent residence, it is not permanent proof. A 10-year green card will need to be renewed every decade. It can take over six months to replace a green card, and USCIS allows you to begin the renewal process 6 months before expiration. It is advised to begin this renewal process as soon as you can.
A 2-year (“conditional”) green card cannot be renewed, the conditions must be removed or the immigrant will lose their permanent resident status.
To renew green card, you must file Form I-90. If you are outside the United States and your green card will expire in the next six months, you should file for a renewal card as soon you arrive to the US. If it expires while you are overseas, you will need to contact a US consulate, USCIS office, or port of entry before filing the I-90.