You mean you want to fill out a 2015 return to see how to do it and to see if you come out close to the 2015 return that was filed? You can't use the online version (Online is only for 2016). You would have to buy the 2015 program and install it on a full Windows or Mac computer not a mobile device. You can buy the Desktop CD/Download program here <a rel="nofollow" target="_blank" href="https://turbotax.intuit.com/personal-taxes/past-years-products/">https://turbotax.intuit.com/personal-taxes/past-years-products/</a>
As a first time user of TurboTax, I highly advice you do NOT use the online version of the program. I suggest you purchase the CD/download version that you physically install on your computer. That way, it's easier to start over as you will need to do multiple times in your process of learning the program.With the desktop version, you just close the program, delete the .tax2016 file in the documents\turbotax directory, then open the program and start over. Takes 2 seconds. 5 seconds the first time.
With the online version, it's not the simple (from a first time user perspective). You have to be "in" the tax return, then click the My Account link, select "clear and start over", OK the prompts and that's "supposed" to do it. Doesn't always until after you sign out of your online account and then log back in. You don't need headaches as a first time user learning the program, and that's just one of the many potential headaches you can experience.
Also, the desktop version is significantly easier to navigate than the online version. So that's why I suggest desktop. Once you've learned it and have successfully filed your taxes with it this year (if you choose to do so), then if desired you can always transfer to the online version next year.