If you already filed, you would amend. A child born at any time during the year is considered to have lived with you the entire year.
First, make sure you really need to amend and that your 2020 return was accepted in TurboTax Online or paper-filed through the mail. You can't amend a return that hasn't been filed yet.
When you're ready to amend, find your situation below and follow the instructions.
TurboTax Online
TurboTax CD/Download
