Did you already file a 2015 return? Then you need to amend it to add to it. How to Amend https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
If you need to file an original 2015 return, Online is only for the current year so you have to buy the Desktop CD/Download program here https://turbotax.intuit.com/personal-taxes/past-years-products/
Or you can get the blank forms from the IRS and do them by hand. Either way you will have to print and mail prior year returns.
If I buy the software does that mean I can use it to do both tax years for the cost of the software? What happens t the software in 2017 tax year? Do I have to buy it again?
Sorry no. You have to buy each year separately. It is only good for 1 tax year.
<a rel="nofollow" target="_blank" href="https://ttlc.intuit.com/questions/1901284-do-i-need-to-buy-a-new-turbotax-program-every-year">https://ttlc.intuit.com/questions/1901284-do-i-need-to-buy-a-new-turbotax-program-every-year</a>
I have a business I sold for installment payments in 2015 sole proprietor. no other income besides I am also a partner in a small LLC which I didnt really profit from and have a schedule K1 from that partnership. do I need the most expensive version just to enter that form? If not what version will I need?