I bougth app by accident

I won't ask how you did this but you would have received an electronic receipt, On the bottom is an invitation to report a problem. Report this as your problem.

Having said that, Apps are no more returnable than bars of chocolate you have eaten. Apple will usually refund for genuine mistakes on a one time only basis but that may depend on the value. Expensive, creative apps may be looked into more closely.

Check this link for the steps you need to take: Report a problem with an item purchased from the iTunes Store, App Store, Mac App Store, or iBooks Store - Apple Support