If your e-file is still pending, you won't be able to make changes until the IRS either accepts or rejects it. It is highly likely that the IRS will reject your return due to incorrect input of your stimulus payments. If they don't, it is possible your refund/amount due will be automatically adjusted based on the stimulus money you received in 2020.
If your return is not rejected, you will have to amend your return. To do so, please follow the directions on the How do I amend my 2020 return?
After you click to amend your return, you can follow these steps in TurboTax to reenter your answers:
New - If you filed and claimed the Recovery rebate credit but already received the full Stimulus payments you can call it back and update your return so it isn't delayed