If you used the Desktop software, your return is not on TurboTax Online, it is on the computer you used to prepare your return. The instructions you were looking at is for the Online version.
For the Desktop software, on the first screen when you open the program select “Amend a filed return.”
Amended returns must be mailed. The amendment form will not be available in TurboTax until the end of February or early March.
