After you file

How do I amend my 2017 TurboTax Online return?
You'll have to do this on a computer or laptop, as it involves installing the TurboTax download software. You won't be able to do this on a mobile device.

Sign in at TurboTax.com with the login you used when you filed your 2017 return.
Click Show to open Your tax returns & documents.
Select 2017, and then click Amend (change) 2017 return.
In the expanded instructions, click the links in step 1 (download your .tax file) and step 2 (download TurboTax Desktop 2017).
Install the TurboTax Desktop 2017 software. Win instructions | Mac instructions
Now, open your .tax file in the TurboTax Desktop 2017 software by choosing File > Open Tax Return (Windows) or TurboTax > Open Tax Return (Mac). Browse to the location of your tax file, and then click Open.
Finally, you are ready to amend your 2017 return. Follow the instructions in this FAQ.