If you have already filed your return, you won't be able to make any changes until your return has been accepted or rejected by the IRS. If rejected, you would need to correct any information that caused your return to be rejected as well as make any other changes to your return that you see fit, then proceed to resubmit your return. If accepted, you would need to amend your return. You can follow the instructions in this FAQ here to amend your 2020 tax return. If you have not yet filed your return, you can follow the instructions here to remove a dependent.
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You can still claim someone as a dependent if they made less than 4,300 and lived with you all year. Or is she your daughter? How old? She will have to check the box on her return that says she can be claimed on someone else's return even if you do not claim her.