You will have to wait until the return is either accepted or rejected by the IRS.
If the return is rejected by the IRS you can add the W-2 and refile your return.
If the return was accepted by the IRS then you will have to file an amended tax return using Form 1040X to add the W-2.
The link below has information that you may find useful.
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
