You can downgrade using the Clear and Start Over feature but bear in mind that the different TurboTax "flavors" handle different tax situations and their associated forms.
For example, if you got upgraded to Deluxe because you're itemizing (Schedule A), you can clear and start over in Free Edition, but as soon as you've entered sufficient itemized deductions to exceed your standard deduction, you'll be asked to upgrade to Deluxe again.
Here is a short list of some common tax situations and the online products that now uniquely support them:
Please see below for more details.
https://ttlc.intuit.com/questions/1894467-can-i-switch-to-a-lower-priced-version-of-turbotax-online
https://ttlc.intuit.com/questions/2586254-how-do-i-clear-and-start-over-in-turbotax-online
https://ttlc.intuit.com/questions/1901213-help-with-my-turbotax-fees?jump_to=answer_3301994
