It's super easy, and just a few clicks away.
For Android devices:
Open the Logbook app and select profile&settings from the side menu.
There you'll find the orange "Manage Subscription" button, which takes you to the Google Play Store. There you can quickly and easily turn your subscription off.
Or, here's the link to Google Play to cancel it there directly: https://support.google.com/googleplay/answer/2476088?hl=en
For iOS (Apple) devices:
You can cancel the subscription by going to Profile & Settings. Then tap Membership under the About Me section. It should then take you to the store where you purchased the subscription so that you can cancel it :)
Or, here's how to cancel it within iTunes: https://support.apple.com/en-en/HT20203