If you wish to alter your Billing cycle i.e. from Monthly to Annually then this can be achieved within the Change Subscription screen.
Navigate to: Billing -> Change Subscription
The screen will display your current subscription with its cycle displayed.
Using the purple drop down list, you can select the same plan you are subscribed to now but with a different cycle, complete your order to apply the changes and you will receive a confirmation message saying that the changes have been applied to your account.
Fig 1 - Changing Billing Cycle