Unfortunately, there is no way to downgrade a TurboTax return. You would need to create a new return to go down to a lower 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.
If you have TurboTax Live Assist & Review, see this TurboTax FAQ to learn how to talk to a Tax Expert or book a review: In TurboTax Live Assist & Review, how do I talk to a tax expert?