Unfortunately, once your TurboTax return has been upgraded, there is no way to downgrade. 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.

Please see these TurboTax FAQs for more information on why your return was upgraded:

Why did I have to upgrade my TurboTax Online edition?

Who can file for free using TurboTax?