First, the 1099-SA has nothing to do with HSA contributions. The 1099-SA reports on how much you spent out of the HSA ("distributions"), presumably on qualified medical expenses.
Second, the screen with the title "Let's enter [name]'s HSA contributions" shows the "employer" contribution, but, as you have found, you cannot change it there. That is because it is the amount with code W in box 12 on your W-2. If you were to change it, you would change it on your W-2.
However, you would not change it on your W-2 unless you knew that the employer got it wrong, and then you would ask the employer for a corrected W-2.
Note that the "employer" contribution is the sum of what your employer contributed to your HSA PLUS what you contributed through payroll deduction. Please don't be confused that the payroll amount is a "personal" contribution - it's not, in the eyes of the IRS.
I do not know what to say about 2018 vs 2019 contributions since I can't see your W-2. Are you saying that the code W amount in box 12 on your W-2 is for 2018 and not 2019?
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