If you have purchased a subscription in the app via Apple, you can easily cancel your subscription through your iTunes account.
Please log on to your iPhone or iPad and follow the steps below:
1. Go to Settings > [your name] > iTunes & App Store.
2. Tap your Apple ID at the top of the screen.
3. Tap View Apple ID. You might need to authenticate your Apple ID.
4. Tap Subscriptions.
5. Tap the subscription that you want to manage.
6. Tap Cancel Subscription to cancel your subscription.
You will have access to Readly's full catalog for the remainder of the month that you have paid for.