In order to file the 2014 and 2015 returns, you will need to purchase the desktop download version for each of those years and prepare the return. These cannot be done in the TurboTax online version of the software.
You can find the desktop versions for prior years here https://turbotax.intuit.com/personal-taxes/past-years-products/
NOTE: You cannot e-file prior year returns. You will have to prepare those returns and then print and mail them.