Refunds are handled by whoever processed your payment, so where you ask depends on where you subscribed.
On iPhone
Purchases made through the App Store are refunded by Apple. You can request a full refund on Apple's refund page.
On Android
Purchases made through the Play Store are refunded by Google. You can request a full refund by following Google's instructions.
Good to know
Apple and Google review and approve App Store and Play Store refunds, so the final decision is theirs. Your access usually continues until the refund is processed.