I accidently subscribed to an app and want a refund.

You must first cancel any subscription automatic renewal (if you simply delete an app this does not cancel subscriptions):

Review the information in this support article by clicking on the link to learn "How to cancel a subscription from Apple." - https://support.apple.com/HT202039

• "If you signed up for a free or discounted trial subscription and you don't want to renew it, cancel it at least 24 hours before the trial ends."

• An existing subscription will normally continue until the end of its term and may then renew automatically. Stopping automatic renewal is effectively canceling it. "If you don’t see Cancel Subscription, then the subscription is already canceled and won't renew." The "Expires" date means the current subscription will just run until then.

• Read the link to see what to do about subscriptions that do not appear in your list.

• Canceling a subscription does not automatically start a refund. If you want to do that:

Click on this link to "Report A Problem" and ask for a refund. --> https://reportaproblem.apple.com/?s=6

If Report A Problem does not work for a subscription refund you may have to make a special request. Click here --> https://getsupport.apple.com/?caller=kbase&PGF=PGF51002&category_id=SC0060&symptom_id=23330

Apple's terms of purchase in the USA are, "All Transactions are final" (https://www.apple.com/legal/internet-services/itunes/us/terms.html ), but some countries have different laws so check terms for your country at: https://www.apple.com/legal/internet-services/itunes/. If sales are final then it is Apple's decision if they want to grant a refund. You can always ask, and people do get refunds on a case by case basis, but Apple can also say no.