Let's make sure we are talking about the right thing.
Box 12a, 12b, 12c, and 12d are for reporting various tax items, but they can contain anything and be in any order. Only an item with code W is your HSA contributions.
If your box 12 code W reports an incorrect HSA contribution, you need to get a corrected W-2 from your employer. There is a procedure to submit a "substitute W-2" with different information, but that raises audit risk and should not be used unless your employer refuses to issue a corrected W-2. If your employer has agreed to issue a corrected W-2 but you haven't received it yet, the best thing to do is to get an extension and wait to file your return until the corrected W-2 is received.
