Can i restart my 2020 return from a saved .tax file

Have you already filed that saved file?

I have now.  But this would be useful if I want to check something.  Of course, I understand that an Amendment is required if something actually has to be changed.

You can transfer last year's tax data file (with a filename extension .tax2019) if you're preparing your 2020 return online in TurboTax Live Basic, TurboTax Deluxe, TurboTax Live Deluxe, TurboTax Premier, TurboTax Live Premier, TurboTax Self-Employed, or TurboTax Live Self-Employed. (Free Edition, with or without MAX, doesn't support transferring last year's .tax file.)

• You won't be able to transfer a PDF, it must be a tax data file with a filename extension .tax2019.
• Remove any password protection from the data file before transferring.

@Illini