If you have already e-filed or mailed your return, you will need to wait until the IRS has accepted it. Once they accept it and you have received your refund, then you can amend it to add the additional W-2. If the return gets rejected, you can just make the needed changes and file again.
NOTE: The IRS plans to start accepting electronic returns on January 23, 2017. The amend form will be ready for you in mid-February. Check back at this time if you would like to amend your 2016 return.
See below for reasons you'd need to amend your return:
https://ttlc.intuit.com/replies/3318732
Other Related Information:
If the return was already filed, you will need to wait until it has been processed and you have received your refund or confirmation that any tax due was paid. (If you file an amended return while your first return is being processed, it can cause extended delays for both returns)
How do I amend my 2019 TurboTax Online return?
Amended returns can only be mailed. It is suggested that it be mailed certified with return receipt (or other tracking service) to verify that the IRS receives it.
You can check the status of the amended return here, but allow 3 weeks after mailing.
How do I track my amend status?