I have printed my 2020 return to file by mail but have not sent it yet. Now I find some additional information that must be entered How to I get back to the data entry?

As long as you have not mailed it, there is a way to unlock it so that you can make the changes and not have to file an amendment.   If you have already mailed it, you should wait until it is processed and then file an amendment later.  

If you printed your taxes to mail them in and went all the way through the File section, when you sign in to TurboTax Online you'll see the Way to go! You filed your 2020 taxes and you won't see a way to make changes or switch to E-file.   As long as you haven't already mailed your 2020 paper return, you can unlock the return (through October 15, 2021). 

If you already mailed your return, don't unlock your return. This can cause problems with the government, including refund delays.

1. In TurboTax Online, sign in to your account.
2. Select Tax Home in the left menu.
3. Scroll down to Your tax returns & documents.
4. Select 2020, and then select Add a State (you're not actually adding a state, this just gets you into the right area).
5. Make your changes and then proceed through the filing screens again - you can switch to e-file at this point if you choose to.  

Click on Add a State to let you back into your return.