I haven't filed yet, why can't I change my return?

To access your current or prior year online tax returns sign onto the TurboTax website with the User ID you used to create the account - https://myturbotax.intuit.com/

When you sign onto your online account and land on the Tax Home web page, scroll down and click on Add a state. 

This will take you back to the 2020 online tax return where you can make any changes needed before filing.

That only lets me into state and not federal, I need to fix something on federal.

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@felshaw2002 wrote:

That only lets me into state and not federal, I need to fix something on federal.

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That takes you back to your complete tax return, federal and state.  Click on Federal on the left side of the online program screen when you are accessing the 2020 online tax return.