Since you have already e-filed or mailed the return, you will need to wait until the IRS has accepted that return. 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.
See some reasons you'd need to amend your return here https://ttlc.intuit.com/questions/1908543
Since you have already e-filed or mailed the return, you will need to wait until the IRS has accepted that return. 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.
See some reasons you'd need to amend your return here https://ttlc.intuit.com/questions/1908543