What was changed on the W-2?
If it was something that would have no tax effect then you would not need to do anything.
The amount I paid in was incorrect and not in my favor.
Paid in to what??????? Are you referring to box 2 federal income tax withheld? Your employer reduced the amount in box 2?
Yes. I guess there was an error.
prepare an amended return showing the correct info. If your tax changes, file the amended return.