To cancel your subscription to Declutter The Mind, you must go into your Settings within your iOS device and manually cancel the subscription.
To do this, go to Settings > [your name] > iTunes & App Store. Tap your Apple ID at the top of the screen, then tap View Apple ID. You might need to sign in with your Apple ID. Scroll to Subscriptions, then tap it. Tap the subscription that you want to manage. Click Declutter The Mind and click Cancel Subscription.