I would like a refund

Are you talking about a purches in the iTunes Store or the App Store? Strictly speaking all purchases are final (after all, you can't very well return them): please see this page for information on requesting a refund: Request a refund for an App Store or iTunes Store purchase - Apple Support

However Apple have been known to be sympathetic in cases of actual error, so it's possible that they may allow you a refund as a one-off concession: but please be aware that they are under no obligation to do so: go to iTunes Store - Contact Us - Official Apple Support

Alternatively go to the page suggested in the Help page linked to above: Report a Problem

You can't purchase something that you have already purchased. If you already bought it, it would be in your purchase history and available for download again, if it wasn't already on your device.

Request a refund for an App Store or iTunes Store purchase - Apple Support