this is a situation without a perfect answer. technically you should amend, however, this change is small and the IRS may not bother you if you do not amend.
if you amend you will not be able to e-file 2020 so it could take 6 to 8 months, maybe longer for the IRS to process it. until it is processed you'll not be able to find out its status. I would also suggest that you send by a method where you get proof of delivery.
as to 2021 if it's been fully processed you can amend it at this time and e-file. it is unknown if the IRS will hold up processing until it gets the 2020 amendment
with the 2021 return the tax was due 4/18/2022 so not only will you owe the tax but the IRS may charge penalties and interest.
if you have a state return that further complicates the matter.
