How do i modify a 2021 1040 that is on extension and has not been filed?

To change a return If you haven't mailed or efiled yet, see

https://ttlc.intuit.com/community/tax-topics/help/i-want-to-change-a-return-i-ve-printed-but-haven-t-mailed/00/27616

If you haven't efiled or mailed your return or it rejected you can go back in to finish and change anything you want.  Do not click on Amend!

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

From Tax Home (the first screen you see after you sign in), scroll down to Your tax returns & documents. You may have to select Show to open the drop down.

Select 2021, and then select Add a State (you're not actually adding a state, this is just the workaround).

Then when you are ready to file you can try to switch to efile.

  https://ttlc.intuit.com/community/e-file/help/how-do-i-change-from-mail-to-e-file-in-turbotax-online/00/26985