If you have already filed, you have to wait to see if your return is rejected or accepted.
If your return is rejected by the IRS, you can add that W-2 and correct other errors, then re-file your return.
If your return is accepted, you'll need to amend your return. Please note that the amended form will not be ready before mid-February.
To amend your return, please follow the instructions in this TurboTax Help topic.
If you file the amended return before April 15, you won't owe any penalties or interest on the extra amount you may owe.
My return form was accepted as soon as I clicked submit. I realized my mistake and I have tried to find out how to fix it.
@janetsopp If your Efile has been Accepted by the IRS, wait until you receive your Refund (or your payment has processed) before submitting an Amended Return to add another W-2.
An Amended Return does not 'catch up' with your original return and correct it. It is processed separately and takes several weeks to complete.
Click this link for instructions on How to Amend Your Return.