Are you asking about using your 2019 earned income instead of 2020's to calculate your 2020 EIC and ATC? If your 2020 AGI was too high, you would not have been offered this option.
It sounds like you want to amend so you can use your 2019 income to calculate the Earned Income Tax Credit, or the Child Tax Credit. For instructions on how to amend, go to:
Once you have started the amended return, type "earned income credit" into search, and the program will take you to that section.
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