I just completed my 2020 return using the offline version of TurboTax. Can I upload the data from that return into TurboTax online,
Unless things have changed that I'm not aware of, understand that you can not upload the .tax2020 file from the desktop version of TTX 2020 to your online account and then import from it with the online version of 2021. When you start the online version of 2021 it will give you the option to import the data from the .tax2020 file on your computer.
