The look back to use the 2019 AGI is only available for the Earned Income Tax Credit and the Child Tax Credit.
The 2020 federal tax return still uses the 2020 AGI for all other calculations on the 2020 tax return.
Once a tax return has been filed it cannot be changed, canceled or retrieved
You will have to wait for the IRS to start accepting or rejecting 2020 tax returns on February 12, 2021
If the return is rejected you can make the necessary changes and e-file again.
If the return is accepted you will have to amend your original tax return. An amended return, Form 1040X, can only be printed and mailed to the IRS if the original return was not e-filed. The IRS will take up to 16 weeks or longer to process an amended tax return.
Before starting to amend the tax return, wait for the tax refund to be received or the taxes due to be paid and processed by the IRS.
See this TurboTax support FAQ on amending a tax return originally completed and filed using the 2020 TurboTax online editions - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/01/27577
