If you signed onto the 2017 online account using the exact same user ID you used to create the 2016 online account then the 2016 AGI would have transferred to the 2017 tax return.

Otherwise -

To access your prior year online tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/

Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show.  Click on the Year and Click on View adjusted gross income (AGI)