First, open up iTunes on your PC or Mac. You can't do this on the device itself. Click 'Store' on the file menu, and choose 'Sign In...'
Enter your Apple ID and password, then go back into 'Store' and choose 'View My Apple ID'.
You'll see your 'Account Information'. Find 'Purchase History' and click 'See All'.
You'll now get a complete history of every purchase you've made on iTunes. Every song, every audiobook, every app, and every in-app purchase.
First, though, click 'Report a Problem' at the bottom of the page.
Now, every item in your purchase history has a 'Report a Problem' link to the left of its price. Find the app or in-app purchase you want to request a refund for, and click the new link.
We'll try and get a refund for Super Hexagon because we're so embarrassingly terrible at it (just kidding).
You'll be presented with a window that lets you send a refund request to Apple.
Choose a problem from the drop-down box (like "I inadvertently purchased this app", "this app does not function as expected", or "this app is not compatible with my device") and explain yourself in the comments box.
Now, hit 'Submit' and hope for the best.
Apple will get back to you via email and give you the verdict. Good luck!