Email [email protected]. Tell us which app, what you expected, and what happened instead. A screenshot helps a lot.
We usually get back to you within a day or two, and faster if it's a bug we already know about.
Purchases go through Apple, so refunds do too. You can request one at reportaproblem.apple.com. We can't issue them ourselves, but we're happy to help if you get stuck.
Open the app, go to Settings, and tap Restore Purchases. It only works if you're signed in with the same Apple ID you used to buy it, which trips people up on a new phone or a family device.
On your iPhone, open Settings, tap your name, then Subscriptions. Pick the app and tap Cancel. You keep access until the end of the period you paid for.