How do I request a refund for an app purchase on iPhone?

Do you mean for a purchase in the App Store? If so all you can do is request a refund using the link below. In most cases all transactions in the App Store are final, but at times Apple may provide a compassionate refund. It just should never be expected though.

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

If you got an email receipt and there's a 'Report a Problem' link in it, use that; otherwise, click here and ask the iTunes Store staff for a refund.

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