I already filed my 2020 return, afterwards I realized I could of used my 2019 return if I made less income in 2020 and received a bigger refund how do I fix this

That look back to 2019 only applies to the Earned Income Tax Credit and the Child Tax Credit.  Not the 2020 federal tax return per se.

Ok so I can amend my 2020 return and use my 2019 earned income credit 

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@Bettylou45  wrote:

Ok so I can amend my 2020 return and use my 2019 earned income credit 

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Only if your 2019 AGI was more than your 2020 AGI would it be necessary to amend the 2020 return to see if you get an increase in the Earned Income Tax Credit for tax year 2020.