Unfortunately Apple does not give developers any control over your subscription or billing. However, Apple makes it pretty easy to manage your RoboKiller subscription from your iPhone. From your iPhone:
- Go to the “Settings” app on your iPhone
- Scroll down to "iTunes & App Store"
- Tap your Apple ID at the top of the screen
- Tap "View Apple ID". You may need to sign in or use Touch ID.
- Tap "Subscriptions"
- Tap RoboKiller
From here you can manage your RoboKiller subscription.