To change your billing cycle, login to your account and got to "Control Panel" ----> "Payment Details". After that click on the "Change Plan" button. A page will open which will have the option to change billing cycle. After you have made the changes, click on "Next" button to complete the remaining process.
Note: Only the admin of the account can change the billing cycle. In case you are not the admin of your account, you are requested to contact your account administrator.