How do you file back taxes (2015) with turbo tax?

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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, sign,  and mail the return(s)

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

  (if you want to prepare on paper for free:  )

You can download the Forms and Instructions from the IRS web site:

https://www.irs.gov/forms-pubs

Remember to prepare your state return as well—if you live in a state that has a state income tax.