When you log in to TurboTax, and click on the prompt to amend, the program will reopen. You will see a screen like the screenshot below. Since you haven't mailed yours in, you will need to select the third button, "No, I haven't filed the amendment yet. I need to make more changes first." You may then make changes to your 1040X, reprint, and mail when you're ready.
Please reference
this
TurboTax FAQ
for how to amend if needed.
