I've downloaded the TurboTax Desktop 2021 for Windows and also downloaded my .tax file for 2021, but the return is not listed so I can't amend it. I've tried many times.

First, make a backup copy of the *.tax2021 data file in case you mess it up when preparing the amended return.  Name it something that sets it apart and store it safely.

Place the one you intend to work on in your Documents\TurboTax folder.

• Launch TurboTax 2021. (assuming you've installed and activated it)
• You will get to the main welcome screen.  See first image below (image is from another year, but should be similar).
• Click bottom left button to begin the amended return.
• On the next screen if TurboTax 2021 does not find the tax file you placed in your Documents\TurboTax folder, then click on "Find Tax File" to browse the file directory and point TurboTax to it.   See second image below.

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If you need further help, you can ask in this forum, and someone may be able to help you here, or here's how to reach TurboTax Support:

Support hours are 5AM-9PM Pacific, every day this time of year.

FAQ: What is the TurboTax phone number?
https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632