If you want to complete, print, sign, date and mail the 2020 Form 1040-X federal amended tax return to the IRS then use the form on the IRS website - https://www.irs.gov/forms-pubs/about-form-1040x
If you want to e-file the 2020 Form 1040-X then you will have to wait for it to be available in the TurboTax program, 03/25/2021.
