I inadvertently purchased an application
How to report an inadvertently purchase?
iOS 6.0.1, reporting an inadvertently purchase
All purchases are considered final, but you can try contacting iTunes support and see if they will refund or credit you :http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption