You cannot open a .tax2015 file with the 2016 software. Did you also save it as a pdf file? If so, you can view that PDF file along side your current tax return results to compare and contrast them.
TurboTax online is only for the current tax year. To do a 2015 tax return summary you would have to buy the CD or download
TurboTax software for 2015. You can purchase a download at the following
link.
http://turbotax.intuit.com/personal-taxes/past-years-products.jsp
You might get a better price from a reputable online seller that still has
TurboTax for 2015 in stock.
I hope this information provides you with some clarity.
Best of luck!