Please see the below TurboTax Help Article for assistance with amending a return.
How to amend (change or correct) a return you've already filed
To assist with claiming the Recovery Rebate, please see this TurboTax Help Article: How do I claim the Recovery Rebate Credit in TurboTax?
This didn't work .. I'm having the same problem and it only allows you to amend your state taxes using the online version.
i amended return for recovery rebate credit when will i know if accepted for third stimulus payment
Track the status of your amended return with the IRS Where's My Amended Return? tracking tool.
Wait three weeks for the amended tax return to appear in the tracking tool.
Allow at least 6-8 weeks for the IRS to process your amended return.
See also here.
@alvarezjun