More info is needed. TurboTax has 2 products with "Premium" in the name. I'll ask some questions to see which one you mean.
Are you using Online TurboTax? If so, are you using the Premium Edition to prepare your taxes, and are you trying to downgrade to Deluxe?
Or are you referring to Premium Services bundle, which is an extra $45 fee California filers can encounter when they try to pay their product fees out of the Federal refund? If so, we can tell you how to remove that and pay upfront by credit/debit card.