Since you have already e-filed or mailed the return, you will need to wait until the IRS has accepted that return. (Do not add/change anything on a filed return that has not been accepted) Once they accept it and you have received your refund, then you can amend the return to add that additional W-2. If it gets rejected, you can just make the needed changes and file again.
NOTE: The amend form will be ready for you in mid-February. Once amend form is ready, you can go here https://ttlc.intuit.com/questions/1894381 and get instructions for amending that return.
And if you then need steps on how to enter that W-2, you will find those here https://ttlc.intuit.com/questions/2866494
See some reasons you'd need to amend your return below: