Since you did not file your original return with TurboTax, in order to amend your return with TurboTax you need to enter all your original information into the program exactly as it was when you first filed the return (mistake & all). Then you need to "trick" the program into thinking you filed the return, by choosing to file by mail. Save your return.
Once that is done, start the program & then choose to amend your return. Amended returns must be mailed.
How to amend (change or correct) a return you already filed
https://ttlc.intuit.com/replies/3288565
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As far as the 1040-X form, if you enter the information manually, Column A you enter all your numbers reported on your original return, Column B enter any amounts that changed (increased or decreased), the Column C is just the difference between A & B
