Unfortunately, there is no way to downgrade a TurboTax product once you have upgraded it. You would need to create a new return in the version you want and then re-enter your information.
To start a new return, select “Start a New Return” from under “My Returns” in the left side menu. Once you have started your new return, you can select “View All Tax Returns” from under “My Returns”, and delete the unwanted return by clicking the small Trash Can next to it.