You can cancel your subscription anytime by tapping the settings icon inside the app -> scroll down to “Manage Subscription” and you will be redirected to the subscription management page.
This link should work too (Only from iPhone or iPad):
iOS developers cannot issue refunds since we don’t have direct access to your payments.
You can request a refund from Apple if you believe you shouldn’t have been charged. There should be a link to do so in the email receipt you got from Apple.
Note: Uninstalling iOS apps does NOT automatically cancel your subscription. This has to be done manually as described above.
Please let us know if you have further questions!