You don't correct your original tax returns, or file a corrected tax return. You file an amended return on a special form, Form 1040-X. See the following FAQ for instructions.
How to amend (change or correct) a return you've already filed
You can file the two amended returns at the same time. You don't have to wait for 2020 to be processed before you file 2021. But each year has to be filed separately.
You will not be able to e-file the 2020 amended return. You will have to print it, sign it, and file it by mail. If you e-filed your original 2021 tax return, you should be able to e-file the 2021 amended return. You can use Direct Pay on the IRS web site to make the payment.
If you filed state tax returns, you might have to amend those as well.
