Late W-2 showing up after I already filed and everything was accepted. What can I do?

If your return was already accepted, you will have to amend your return to add the W-2 you just received.   The amendment software will not be ready to use until later this month, see Form 1040X in this chart.   

Go back into your tax return and change it back to the way it was when you filed it.   When you do the amendment, you need to start with the return exactly as you filed it the first time.   You have 3 years to do the amendment, so don't rush it.    The important thing right now is to go in a remove the W-2 you added after you filed.    Don't add the W-2 until you are ready to do the amendment.   As of today, the Form 1040X is scheduled to be ready on 02/24.