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:
We've had to temporarily disable the link that is in the Profile & Settings that will directly open the section in iTunes to cancel the subscriptions, but there is still a way!
Here's how to cancel it within iTunes: https://support.apple.com/en-en/HT20203
*Unfortunately, our support team cannot stop your automatic payments for you - you'll need to follow the steps above if you wish to no longer be subscribed to Pro, even if we have deleted your account. This is because the agreement for the payment was established between you (the user) and the service (Google Play or Apple iTunes) rather than through us directly.*