How can I extend my I-551 stamp?

You should request for an extension as soon as possible. Get in touch with the nearest U.S. consulate for consultation and assistance. You may seek to obtain an I-131A letter from the consulate.

You will need to contact the U.S. embassy for a travel letter.

Under normal conditions, you can call the USCIS Contact Center within 30 days of the expiration of your I-829 extension receipt (valid for 18 months) or your one-year I-551 extension stamp and USCIS will schedule an Infopass appointment for a new stamp. Currently, USCIS offices are closed to the public. If your stamp will expire in 30 days or less, you could try to call USCIS and ask for an infopass appointment sometime in May. Hopefully, by then USCIS offices will be reopened. If not, you will have to call again to reschedule. If you have an urgent need to leave the United States and do not have a valid extension stamp to return, you will have to contact the U.S. embassy or consular office in your home country and complete Form I-131A travel letter/permission to board the airplane back to the United States as a returning lawful permanent resident. Obviously nonessential travel is not recommended at this time and even if you do have an urgent need to travel, you likely will be subject to some sort of quarantine upon arrival at your destination.

You will have to go to the consulate and ask for a I-131A boarding letter? It is not so easy to get with consulates on limited staffing.

File an I-131A and get a boarding foil from the U.S. consulate to return to the U.S.