Once you pay for the upgrade to the 2017 edition of Plus you will have access to all the prior year online tax returns through October 2019. Paying for the upgrade does not mean you must file your 2017 tax return now. You can always sign out and return at a later date to complete and file the 2017 tax return.
If you did not pay for the one of the 2016 online editions of either Plus, Deluxe, Premier or Self-Employed you will need to start a 2017 tax return using the online Free editon and create the account with the exact same user ID you used to create the prior year online account - https://turbotax.intuit.com/personal-taxes/online/
After starting the tax return with the Free edition you will need to upgrade to the Plus edition and pay for the Plus edition so you can access the prior year online tax returns.
To pay the TurboTax online account fees by credit card, without completing the 2017 return at this time, click on Tax Tools on the left side of the online program screen. Then click on Print Center. On the next page, to pay by credit card, click Pay Now. On the next screen just click on the Continue button. The next screen will show a review of your Fees, click on Continue. Complete the next screen to pay by credit card and click Continue.
See this TurboTax support FAQ for accessing prior year tax returns - https://ttlc.intuit.com/questions/3404575-how-do-i-access-my-prior-year-2016-return
See this TurboTax support FAQ for when access expires - https://ttlc.intuit.com/questions/3355524-when-does-access-to-my-prior-year-turbotax-online-returns-...
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 Download/print return (PDF)
You can also get free transcripts of your federal return from the IRS - http://www.irs.gov/Individuals/Get-Transcript
For a fee of $50 you can get a complete federal tax return from the IRS by completing Form 4506 - http://www.irs.gov/pub/irs-pdf/f4506.pdf
