Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello
That's because it is not a switch, but rather, you just subscribed to two plans.
In your Microsoft account https://account.live.com > Subscriptions. Click "Manage" for the personal plan, then cancel it.
Get in touch with support to make sure you get refunded the cancelation. Open the "Get Help" app on your computer (just search for it in the Start menu). or go to https://support.microsoft.com/contactus, Type "Billing" and press Enter. On the next screen, click "Contact support" at the bottom of the page, then choose "Microsoft 365" > "Manage my subscription". You’ll then see the option to chat with a Microsoft support agent who has access to your account to help review the issue.
Also just a reminder - The premium plan is only at this price for the first year, once renewed next year it will be back to regular price. If you want to downgrade at that time, make sure you also do the same, cancel the plan, then subscribe to a cheaper plan.