You renew your green card by filing Form I-90 Application to Replace Permanent Resident Card.

While your renewal is pending, you can request a stamp in your passport I-551 stamp if you urgently need proof of status.

If your card is already expired, you can still apply to renew but do it as soon as possible.

If you are eligible for U.S. citizenship, you might consider applying for naturalization instead of renewing your green card again.