Your app subscription is managed and renewed by the app store you purchased it from (Google Play for Android devices and the Apple App Store for iOS devices).

To cancel your subscription you need to use your device to access your app store account.

On iOS devices go to Settings > AppleID and choose Subscriptions
This will show a list of your current and expired subscriptions. Touch the Warmshowers subscription and you should be able to cancel it.

On Android open the Google Play app, then touch your profile icon (in the top search bar).
Touch Payment and subscriptions, then choose Subscriptions. You should see the Warmshowers subscription listed and if you touch it you should see the details of the subscription together with the option to cancel it. 

Alternatively, you can use the "Manage Subscription" option from the menu within the Warmshowers app, which will take you to the correct location on your device to allow you to cancel the subscription.