To upgrade your current subscription to an annual subscription, follow these steps:
How-to for iOS:
Tap into your Monthbook or Monthly Mini subscription
Tap the Settings (gear icon), found in the upper right corner
Tap "Manage subscription"
Under the "Billing Cadence" section, tap "Billing Frequency"
Tap the "Pay Annually" option
Be sure to tap "Save" and then "Done"
How-to for Android:
Tap into your Monthbook or Monthly Mini subscription
Tap the Settings (gear icon), found in the upper right corner
Tap "Manage subscription"
Under the "Billing Cadence" section, tap "Billing Frequency"
Tap the "Pay Annually" option
Tap "Save" and then tap "Done"
Learn how to upgrade to an annual subscription on web here.