If you already paid TurboTax for the 2017 program, you'll need to contact TurboTax Customer Support to request a free download of the 2017 Desktop (CD/Download) program. Prior-year returns can't be e-filed. They must be printed and mailed.
Once you have the Desktop program downloaded and installed on your computer, see the instructions below for finishing up a prior-year return you started in TurboTax Online.
https://ttlc.intuit.com/replies/4186197
*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***