You can switch your subscription from monthly to annual by canceling your current mobile subscription and then re-subscribing for an annual subscription.
To cancel your current mobile subscription, you will need to cancel your auto-renewal through your device.
Apple App Store
You can cancel through the Apple subscription account settings on your iPhone or iPad, or in desktop Apple on a computer. Please see this support article from Apple for details on how to manage a subscription: http://support.apple.com/kb/ht4098
Google Play
To manage your subscription, either open the Google Play Store app and then select 'My Apps' and look for the Subscriptions tab, or go to the Google Play Store website, click 'Account' in the left-hand menu, and scroll down to the Subscriptions section.
βAnnual to Monthly
For mobile we, unfortunately, do not have any way to reduce your annual subscription to a monthly subscription. You can cancel the subscription on your device following the instructions above, but since you've already paid, even if you cancel you'll still have access to the premium content until the annual subscription ends next year.
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