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:
