To file a 2020 amended return, you would now need the 2020 Desktop software, or if you filed a 2020 return with TurboTax, you can use the version you choose for 2020 (Online or CD) to do the amendment .  You cannot use the 2021 software to file a 2020 amendment. 

 If you used TurboTax Online, simply log in to your account and select “Amend a return that was filed and accepted.” 

If you used our CD/download product, sign back into your return and select “Amend a filed return.” You must file a separate Form 1040-X for each tax return you are amending.

You can also prepare an amended return by manually enter the information.

Federal 1040X form:

https://www.irs.gov/pub/irs-pdf/f1040x.pdf

Federal 1040X instructions:

https://www.irs.gov/instructions/i1040x

Instructions for Form 1040-X (09/2021)

https://www.irs.gov/instructions/i1040x