If your return has already been accepted by the IRS, you would wait until after you have received your tax refund and then Amend your tax return using Form 1040X. Amending instructions will be available - in late January or early February 2018.
If your return gets rejected or has not been filed, you can go back and add the additional W-2 to your return.
To add a w-2:
- Click Federal Taxes
- Then click Wages & Income
- On the Here's your W-2 info screen, click + Add another W-2