Follow the steps below on how to do it:
STEP 1: In your portal, click the more button of the subscription that you wish to change the billing period of and select Convert to Annual Subscription.
STEP 2: A pop-up window will appear. Do not forget to apply a promo code (if you have any) and tick on the check-box to confirm that you agreed on the terms and conditions. Click on the Convert to Annual Subscription button to finish.
Notice that your organization's billing period will change from Monthly to Annual. This change will reflect in your account in real time.