You flat out can not. The online version of Turbotax can only be used for the current tax filing year, which at this time in 2019.
If you need to file a prior year return, then you have to use the CD version of TurboTax that you physically install on your Windows or MAC computer. You can purchase TurboTax 2016, 2017 and 2018 directly from TurboTax at https://turbotax.intuit.com/personal-taxes/past-years-products/
You flat out can not use TurboTax to file a 2015 or older tax return. TurboTax only supports their software for the current tax filing year and three years back. So even if you can obtain the CD version of TTX 2015 or older, you will not be able to get the *required* updates and critical fixes for it. That means your tax return will be wrong and TurboTax will *NOT* honor the 100% accuracy guarantee for the 2015 tax year or older.
Finally, you can not e-file a prior year tax return. The IRS says so. You have to print it, sign it and mail it to the IRS. Additionally, each year's tax return must be mailed in a physically separate envelope for each tax year.
If you also need to file past year's state tax returns, then the same criteria applies.
