Your KAYA PRO subscription is managed through your app store, not through the KAYA app itself.
iOS: Go to Settings on your device, tap your name at the top, then tap Subscriptions, and select KAYA PRO to manage or cancel. Apple's guide to managing subscriptions has more detail if needed.
Android: Open the Google Play Store, go to Payments & Subscriptions, then Subscriptions, and select KAYA PRO to manage or cancel. Google's guide to managing subscriptions has more detail if needed.
KAYA PRO refunds are not guaranteed once a payment is made, so if you plan to cancel, it's best to do so before your next billing date. If you have already been charged and want to request a refund, see "Can I get a refund for a KAYA purchase" for the current steps for iOS and Android.