Using TurboTax you will need to purchase, download and install on a personal computer each tax year, 2015 and 2016, from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
Prior year tax returns can only be printed and mailed, they cannot be e-filed. When mailing multiple tax years use a separate envelope for each year. Consider using a tracking service, return receipt required, from the US post office so you will know that the IRS received the tax returns.
For tax year 2017 you can use either the TurboTax desktop CD/Download editons or the online editions. The 2017 tax return can be e-filed up through October 15, 2018. After that date the 2017 tax return can only be printed and mailed.
Go to this website for the 2017 desktop CD/Download editions - https://turbotax.intuit.com/personal-taxes/cd-download/
Go to this website for the 2017 online editions - https://turbotax.intuit.com/personal-taxes/online/
