If the return was already filed, you will need to wait until it has been processed and you have received your refund or confirmation that any tax due was paid. (If you file an amended return while your first return is being processed, it can cause extended delays for both returns)
If the IRS would reject your return for any reason, then you would be able to go in and make any necessary changes to your tax return and resubmit.
How do I amend my 2019 TurboTax Online return?
Amended returns can only be mailed. It is suggested that it be mailed certified with return receipt (or other tracking service) to verify that the IRS receives it.
You can check the status of the amended return here, but allow 3 weeks after mailing.
How do I track my amend status?
Please refer to the below TurboTax FAQ to determine if filing an amended return is necessary as well as instructions on how to amend your tax return:
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/27577
Please note the Form 1040X is currently scheduled to be released on February 20, 2020.