If you have a 2015 W-2 that was not included with your original 2015 tax return, you will have to amend your 2015 return and put that W-2 into it. Do not include the dependent that you do not want on your 2015 tax return. If you used Turbotax to file your 2015 tax return and want to amend it to include that W-2, follow the instructions in the LINK below.
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
For your 2016 tax return, just fill it out as you ordinarily would and include the dependent that you want included on your 2016 tax return. People change dependents all the time. For example, a divorced couple alternate years in claiming their children. One year the ex-wife might claim the child and then the next year the ex-husband may claim the child. You do not have to claim the same dependent consistently through time if the situation changes.
