Each tax year is a separate filing.  Using TurboTax each tax year is a separate program.

The TurboTax online editions are only for tax year 2016 - https://turbotax.intuit.com/personal-taxes/online/

To complete a 2015 tax return using TurboTax you would need to purchase and download one of the 2015 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/

A 2015 tax return can only be printed and mailed, it cannot be e-filed.