To upgrade your current subscription to an annual subscription, follow these steps:
For iOS:
From the Chatbooks app, on the Home page tap into your Monthbook subscription.
Tap into the Settings (gear icon) in the upper right corner.
Tap "Manage subscription."
Under "Billing Cadence" tap "Billing Frequency"
Tap the "Pay Annually" option
Be sure to tap "Save" and then "Done"
For iOS: How to Upgrade to an Annual Subscription
For Android:
From the "My Books" tab in your Chatbooks app, tap into your subscription.
Tap the Settings wheel, found in the upper right corner
Tap "Manage subscription"
Under "Billing Cadence" tap "Billing Frequency"
Tap the "Pay Annually" option
Tap "Save" and then tap "Done"
Learn how to upgrade to an annual subscription from Chatbooks.com here.