Each version of TurboTax only has the forms & information for one tax year. So you can do one form for 2018, but if you have to fill out forms for previous years, you would have to add them to the returns for those years.
This TurboTax FAQ has more information on how to ReFILE your return: I've filed my tax return. Can I still make changes?
"but if you have to fill out forms for previous years, you would have to add them to the returns for those years."
So how do you do that i.e. add them to returns for those years, do we then have to paper file the return in this case for 2018 with say 2017 and 2016 (previous years) T746s and then send it all in Paper copies. or have to send in T1adj for previous years. It is not clear, how to do that all electronically and send it thru netfile. There must be a way to do that electronically and efficiently.
This TurboTax FAQ has more information on how to change your return: I've filed my tax return. Can I still make changes?