I mistakenly subscribe to an app and want refund

Firstly you should cancel the subscription: please see View, change, or cancel your subscriptions - Apple Support

Strictly speaking all purchases are final (after all, you can't very well return them): please see this page for information on requesting a refund: Request a refund for apps or content that you bought from Apple - Apple Support

However Apple have been known to be sympathetic in cases of actual error, so it's possible that they may allow you a refund as a one-off concession: but please be aware that they are under no obligation to do so: go to https://reportaproblem.apple.com/?s=6