You would have to purchase the Desktop software to prepare a prior year amended return. It cannot be done online. It would also be rather difficult since you did not prepare your original return in TurboTax. You would have to enter your information from your original return exactly the way it was when you filed. Then you need to trick the program into thinking you filed your original return with them, choosing mail in your return.
Then you can prepare an amended return. You can purchase the software at the following link.
https://turbotax.intuit.com/personal-taxes/past-years-products/
You can also manually prepared an amended return.
Federal 1040X form:
https://www.irs.gov/pub/irs-pdf/f1040x.pdf
Federal 1040X instructions:
https://www.irs.gov/pub/irs-pdf/i1040x.pdf
If you need to amend a State return, then you can check your State's website for their forms.
