Each tax year is separate. You cannot put another year's W-2 on a 2016 tax return, which is what everyone is preparing right now. If you forgot to include a W-2 from 2015 on your tax return last year, then you have to amend your 2015 tax return.
https://ttlc.intuit.com/questions/2952222-how-do-i-amend-the-2015-taxes-i-filed-with-turbotax-online
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