You need to recall the source of the download last year (Amazon? Staples? TurboTax?) and download it again. You'll also need the authorization code given you at that time.
However, if all you're doing is your 2021 taxes, you don't need the 2020 program, you just need the 2020 tax file (.tax) on your computer so TT 2021 can access it. You'd only need the 2020 program to amend the 2020 return, or perhaps to see certain details that aren't evident in the .pdf copy you made.
