So you just have a pdf of the return?

You'll want to recreate the return in Basic,make it look exactly like the original.  Then open and activate (put the year in at the top) the 1040X then go back and make the changes that you're amending for.

And if you are amending the State too, you may need to also 'activate' the State amended form (in addition to 'activating' the 1040X) BEFORE making any changes to the original return .