The only way to downgrade your product is by using the "Clear and Start Over" feature. This is available only if you haven't yet registered or made payment for your TurboTax product. Keep in mind, however, that ALL of your data already entered will be erased. And depending on your individual tax situation, you may still be required to upgrade to Deluxe in order to file your return. I'm linking instructions below.
How do I clear and start over in TurboTax Online?