-If you want to file your 2015 tax return with TurboTax,  you will need to purchase the 2015 TurboTax program to do so.  See-https://turbotax.intuit.com/personal-taxes/past-years-products/ 

-If you do not want to pay and just want to download IRS forms and do it manually, see:

 2015 tax forms

-As IRS only allows e-fling for 2016 tax returns, you have to print out your 2015 tax forms and submit by mail.