If you need to upgrade, TurboTax will give you the opportunity to do so if you enter something that is not supported by your current version.
Unfortunately, there is no way to downgrade a TurboTax Online return. You would need to create a new return to switch 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.