If you have not already filed, you can return to your TurboTax account and enter the additional W-2 by entering "w-2" in the Search box located at the upper right of the program screen. Click on "Jump to w-2" and select the blue +Add W-2.
If you have already filed you will need to wait until the IRS either rejects or accepts your return. If your return is rejected, you will be able to go into your account and make the necessary changes and re-submit your return. If the IRS accepts your return, you MUST wait until it has been fully processed and you receive your refund or have paid your balance due. You will need to prepare an amended tax return on a 1040X and mail it in. You cannot e-file an amended return. They have to be mailed, and it takes about 16 weeks for the IRS to process an amended return. In the meanwhile, DO NOT go in and make any changes to your return. 3 weeks after you mailed your amendment, you can start tracking its progress at the IRS Where's My Amended Return? site. Instructions can be found here: https://ttlc.intuit.com/replies/5114064