At "Let's enter X's HSA contributions" the "2017 employer and payroll contributions" amount auto-filled is 2X the $ on my W-2 Box 12, Item W. How do I correct this value?

You cannot correct the amount on the first line on that screen ("Let's enter your HSA contributions").

Instead, you must return to the W-2 and correct it there.

It's difficult for us to say, since we cannot see your tax return, but evidently TurboTax has your code W amount twice in memory.

I suggest you clear the W-2 and HSA information and start over.

1. make a copy of your W-2(s) (if you don't have the paper copies)

2. delete your W-2(s) (use the garbage can icon next to the W-2(s) on the Income screen

*** Desktop***

3. go to View (at the top), choose Forms, and select the desired form. Note the Delete Form button at the bottom of the screen.

*** Online ***

3. go to Tax Tools (on the left), and navigate to Tools->Delete a form

4. delete form(s) 1099-SA (if one), 8889-T, and 8889-S (if one)... Note, if you notice that there is still a W-2 form lingering, please delete it also.

5. go back and re-add your W-2(s), preferably adding them manually

6. go back and redo the entire HSA interview.