How to add a 2015 w2

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You cannot add a 2015 W-2 to a 2016 tax return.  If you filed a 2015 return last year, then you need to amend it by including that W-2.  If you did not file your2015 tax return, then you need to use 2015 software or a 2015 tax return form to file a 2015 tax return.

https://ttlc.intuit.com/questions/2952222-how-do-i-amend-the-2015-taxes-i-filed-with-turbotax-online

To file a return for a prior tax year

Only a 2016 return can be prepared online and e-filed.  If you need to prepare a return for 2013, 2014, or 2015 you can purchase and download desktop software to do it, then print and mail the return(s)  TurboTax Software for years prior to 2013 is unavailable, but you can get forms to file from the IRS website.

https://turbotax.intuit.com/personal-taxes/past-years-products

https://www.irs.gov/Forms-&-Pubs