To downgrade, you'll need to clear and start over. Keep in mind that you won't be able to do this if you've already submitted payment or deducted the TurboTax fee from your refund.
Know that each version of TurboTax is designed to handle different tax situations and their associated forms. While you’re welcome to downgrade, you may miss some features — and you’ll be prompted to upgrade when those features are needed.
You can also reduce your TurboTax fees by removing add-ons: