If you have already filed your return, you will need to amend your tax return to add the new W2 you have.
You will need to wait until the IRS has accepted your original return before filing the amendment. If you are to receive a refund, wait until you receive it before filing the amended return. If you owed the IRS money, then wait for your payment to clear.
For TurboTax Online, simply login to your account and select “Amend a return that was filed and accepted.” If you used our CD/download product, sign back into your return and select “Amend a filed return.
You can find out more about amending a return here