An amended return uses the original return as a starting point. To prepare an amended return with TurboTax, you first have to recreate your original return in TurboTax, making sure that it exactly matches the return that was filed, then amend the recreated return.
Some people will tell you that you have to print the recreated return (create a PDF) to make TurboTax think that you filed it. That is not necessary with TurboTax Desktop. I don't know if it's necessary with TurboTax Online.
