No, you do not amend your 2020 tax return. If your spouse receives Social Security for the retroactive payments then you will still enter it on the tax return you file for the year he RECEIVES the SS. He will get a SSA1099 for Social Security received. If there is a lump sum included there will be screens that will walk you through how to claim the SS for any prior year. If you just got the letter, you might not get a check until 2022---so you will not put anything on your tax return until you file your 2022 return. If you somehow receive a check before 2021 ends and they send you a SSA1099 for 2021 then it will go on your 2021 return.