Unfortunately, there is no way to downgrade a TurboTax return. You would need to create a new return to go back to the original product. 

To start a new return, select “Start a New Return” from under “My Returns” in the top left corner. Once you have started your new return, you can select “View All Returns” from under “My Accounts & Returns”. Then delete the unwanted return by clicking the small Trash Can next to it or “Delete” under the return’s name.

FYI: There have been changes made to TurboTax Online. Please see these TurboTax FAQs for more information:

Why did I have to upgrade my TurboTax Online edition?

Who can file for free using TurboTax?