How do I request a refund for an Apple purchase?

You request a refund using this link:

Request a refund for apps or content that you bought from Apple - Apple Support

And cancel a subscription using this one:

View or cancel app subscriptions on iPhone - Apple Support

You haven't described how this happened in the first place, so it is difficult to advise on how to prevent it from happening again and refunds are never guaranteed, so don't just ignore how that happened.

• For a purchase on your device, it requires you to double click the side button with the amount showing on the screen followed by verification with Face ID/TouchID or Passcode. Anyone with your Passcode is authorized to use your device and make purchases, so you may need to change your Passcode.
• Free Trials must be cancelled a day before the trial ends to avoid a charge as seen in the message on the Authorization Screen that requires you to Double Click the side button to accept.
• If you do not see the charge in your Purchase History to request a refund, then it may be an indication that your credit card has been compromised and you should contact the bank of the issued card immediately.
• If someone knows your Apple Account Password, they can possibly make purchases to your account, requiring you to change your Password.
• If you are the Organizer of a Family Plan using Purchase Sharing, then you have authorized members to make purchases to your account. You can turn off Purchase Sharing or add Parental Controls to prevent that from happening.

Here's how to request an app or media refund from Apple (From: Request a refund for apps or content that you bought from Apple - Apple Support - see the article for more detail.) If you bought this in the EU from a third party, see: About purchases not billed through the App Store or the App Store’s In-App Purchase system - Apple Support instead.

1. Sign in to https://reportaproblem.apple.com

2. Tap or click "I'd like to," then choose "Request a refund."

3. Choose the reason why you want a refund, then choose Next.

4. Choose the app, subscription, or other item, then choose Submit.

If you were charged for a subscription that you no longer want, you can also cancel the subscription using the directions in this article --> If you want to cancel a subscription from Apple - Apple Support

Read this support article for information about how long it takes to receive a refund (if you are granted one), and how to check the status of a refund for apps or content that you bought from Apple --> Check the status of a refund for apps or content that you bought from Apple - Apple Support