You will not be able to make any changes until your return is processed. You will have to amend your return once it is accepted by the IRS in order to add the missing W-2. However, If they reject your return, then you will have the opportunity to make any necessary corrections, add anything that was left off, and resubmit your return. The instructions to amend are below:
https://ttlc.intuit.com/replies/3288565