Sorry to hear about the accidental purchase!
Since you purchased your App subscription through the mobile app via iTunes, you will need to contact them in order to request a refund. Only Apple Support can process refunds for purchases made through an iTunes account.
Please contact Apple iTunes Customer Support directly at https://support.apple.com/contact
For purchased made through Google Play, you will need to contact them in order to request a refund. Only Google Play can process refunds made through a Google Play account. You can see Google Plays policy here - https://support.google.com/googleplay/answer/2479637#apps
We also recommend that you cancel your subscription to prevent being charged again next cycle.