How do I down load 2020 turbo tax to a new computer?

You can't transfer the programs.  They have to be installed from the CD or Download.  You don't need to install the prior year programs unless you need to amend that year.  Before you move your tax return files make sure you have saved each year as a PDF file.  So you may want to open each year and do that.  FILE - SAVE TO PDF

What I would do is just copy the whole Turbo Tax folder that is under your Documents.  That should be where the .tax files and PDF files are stored.  Then copy that folder to a flash drive or best yet is to burn it to a CD or DVD and then you will have a backup of them.   Then on the new computer copy the folder (or files) from the flash drive to your Documents folder.  Those can always be at least read by any system, even if they can't be used directly by the program.

Save the PDF for each year twice.

1) Once as the tax return for filing   (this contains just the filing tax forms)

2) Again as the tax return, but with ALL the forms and worksheets (this contains all of your worksheets and shows all  of your entries on some form or another).

The selection of the bare tax return, or the extended tax return with everything is made during the Save-to-PDF process.

And back those PDFs up to a backup USB or CD....every year we get tons of folks begging for access to a tax return on a crashed disk....TTX doesn't have it...only you do when you use the desktop software.