You cannot add another W-2 once you file. First you will need to wait and see if your return is Accepted or Rejected by the IRS. If the return is Rejected you can go back in and fix the mistake without amending. If the return is Accepted you will need to WAIT until you receive your refund and then Amend.
Follow these steps to amend:https://ttlc.intuit.com/replies/3288565