You must have a copy of the original tax return you want to update.
The simplest way is to prepare your correct tax return (using other software not involving Windows) and print it.
Then lay the paper printouts of both returns (correct and original) side-by-side.
You now have all the information needed to prepare Form1040X by hand. It's pretty straight-forward.
The blank form in writeable PDF format and its instructions are available at www.irs.gov.
Mail to the same address you would mail your original tax return.
