At this point in time, it's too early to amend a 2019 return. And you should wait for the original return to finish processing and wait on any Federal refund (if applicable) to arrive first before amending.
The Form 1040X will not be available in Online TurboTax until 2/05/2020.
The amended return must be printed, signed, dated, and mailed. It cannot be efiled.
Here are the instructions to use once the Form 1040X becomes available.
FAQ: How do I amend my 2019 Online TurboTax return?