Hello
Where did you purchase it? If it is directly from Microsoft, use the following method to chat with support, explain to them about the mistake you made. Typically they would understand and process the refund so that you can correctly purchase Win11.
To get help, 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.