You mean for the EIC lookback? What do you mean you have filed but not sent it yet? Did you submit your efile? Or does it say to print and mail it?
The special lookback rule will allow lower income individuals to use their earned income from 2019 to determine their Earned Income Tax Credit and the refundable portion of the Child Tax Credit in 2020, since their lower 2020 income could reduce the amount they are eligible for.
You can look back and use your 2019 income for the EIC credit instead of 2020. See this post https://ttlc.intuit.com/community/taxes/discussion/i-m-at-a-section-asking-me-do-you-want-to-use-las...
If you filed........
You cannot change or add anything on the return that you e-filed. It is too late, just like when you put an envelope in a US mailbox
Now you have to wait until the IRS either rejects or accepts your return. Which won't be until Feb 12 this year. If your return is rejected, you will be able to go into your account and add or change it and resend it. Click on Add a State to let you back into your return..
If the IRS accepts your return, then you have to wait until it has been fully processed and you have received your refund or paid the tax due. THEN you can prepare an amended tax return 1040X and mail it in. You cannot e-file an amended return. Meanwhile, DO NOT change anything on your return until you start the amended return. If you did change or add the missing info then take it out and put your return back to the way it was when you first filed it.
You can check your efiling status here,
https://turbotax.intuit.com/tax-tools/efile-status-lookup/
How to amend an Online return
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/27577
