I missed a w2, but already filed and got my federal return already. What do I do?

Since your original filing was accepted, and you have already received your refund, you will need to file an amendment.

Please reference  this TurboTax FAQ  for how to amend.

Then, to add your W-2 while working in amendment, please reference this TurboTax FAQ.

Please remember:

Three weeks after you mailed your amendment, you can start tracking its progress at the IRS Where's My Amended Return? site.

Before starting to amend the tax return, wait for the tax refund to be received or the taxes due to be paid and processed by the IRS.

• You're free to cash your refund check or spend it once you have it. You don't need to wait for your amendment to finish processing, which can take another 3-4 months.
• If you owe money after amending, you'll just include payment with your mailed amendment form.
• And if your amendment results in a second refund, you'll get a check for the additional amount.

An amended return, Form 1040X, can only be printed and mailed to the IRS, it cannot be e-filed.  

The IRS will take 16 weeks or longer to process an amended tax return.