I need to amend my 2017 return. I've downloaded my .taxfile but it won't open because there isn't an app associated with it. What can I do?

You will have to get the 2017 TurboTax Desktop software.  For more information see the following help article:

How do I amend a 2017 return in TurboTax?

After you install the 2017 desktop software, start the 2017 TurboTax program, then open the .tax2017 file from within the program. Don't try to open it by clicking on the .tax2017 file in Windows.

If your .tax2017 file contains a state tax return, you must install the state program before you can open the file.

To get a refund from the 2017 amended return, you must file the amended return by April 15, 2021. Send it by certified mail with a return receipt so that you will have proof of mailing, tracking, and confirmation of delivery.