See @CatinaT1 's advice above and amend your return.
If you fail to report the missing W-2, however, the IRS will send you an automated Notice of Proposed Increase in Taxes later this year.
They will calculate the tax due (plus interest and penalties) and you can send them a check. No amended return is necessary, so you won't have to upgrade.
The IRS uses this simplified procedure since It is extremely common to omit reporting a W-2 or Form 1099. You file before they arrive, they get lost in the mail, get lost in your house or get tossed along with junk mail.
