You cannot add a W-2 from 2018 to your current return (2019). You would have to amend your 2018 return to add the 2018 W-2 or if you have not filed a 2018 you have to prepare a 2018 return as soon as possible because if you owe for 2018 the interest & penalties accumulate until the taxes are paid in full.
You can purchase the 2018 software from TurboTax at the following link (or check the Internet)
https://turbotax.intuit.com/personal-taxes/past-years-products/
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If you filed a 2018 return with TurboTax you can amend your return to add the W-2, using the 2018 Amendment software.
https://turbotax.intuit.com/personal-taxes/cd-download/amend/ty18
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How to amend (change or correct) a return you already filed
You can amend returns that have been accepted by the IRS. If your return is pending, you won't be able to make any changes until it's accepted or rejected by the IRS. If you don't know your e-file status, we can help you check it.
Select a tax year for specific amending instructions.
Keep in mind: You have 3 years from the date you filed your return (or 2 years after you paid the tax due, whichever is later) to file an amended return, which means you don't have to get it done by the April 15 filing deadline.
