To downgrade, you will have to clear and start over:

https://ttlc.intuit.com/replies/4248526

Once you've cleared your return you should be brought back to the original screen that says, "Hi, ______! This is what we got you last year, lets start your 2016 return!"

Under this section is a hyperlink that says, "see other products". You'll want to choose this. If you start for free, and have information pulled in from last year, you'll be upgraded to our PLUS Benefits program. You can avoid this by letting it run its course and clicking continue once everything is in. After you click continue you'll want to select that you do not want to include this and you'd like to Start For Free.