I purchased app by mistake

Kiaanverma,

Start here:

See your purchase history in the App Store or iTunes Store

For a possible refund, see this:

Get help with an item purchased from the App Store, iTunes Store, or iBooks Store

Good luck.

It's as simple as visiting Report a Problem (Apple's reportaproblem.apple.com website) and finding your purchase in the list and then selecting the reason for your refund. You only have something like 30-90 days to do it, so act fast.