How to reload prior year tax to desktop version after amendment to prior year return

Sorry I don't understand your question.  Reload?  Do you need to install the Desktop program?  Where is your tax return file?  It ends in .tax2021 or .tax2020 etc.   Did you amend online on using the Desktop program?  

I need to amend 2020 tax on desktop program while I am working on 2021 tax on desktop.  I started 2021 with the original 2020 return, so I need to update 2020 for 2021.

Oh you amended 2020 and want to update your 2021 return with the changes?  You would have to manually change 2021 or start a new 2021 return and transfer from the amended 2020.  What needs amending on 2020?  It might not affect 2021.  Or have you already filed 2021 and need to amend it also?