To amend a return done by another company or by hand, you would first have to fill out a new return in Turbo Tax like you were doing it for the first time and hopefully come out with the same results as your original filed return. Then when it exactly matches what you filed and amounts are the same for every line on the 1040 tell Turbo Tax you want to file by mail and print out the original return so TT thinks it was filed, then you can do the amended return.
For a current year return you can use either the Online version or the Desktop CD/Download Software. It would be easier to use the Desktop CD/Download program,
https://turbotax.intuit.com/personal-taxes/cd-download
If you want to amend a prior year return you would have to buy the desktop version of Turbo Tax. You can buy the prior year desktop versions here....
http://turbotax.intuit.com/personal-taxes/past-years-products.jsp
