Should you choose to upgrade to a Premium subscription, payment will be charged to your iTunes account at confirmation of purchase. Subscription automatically renews unless auto-renew is turned off at least 24-hours before the end of the current period. Account will be charged for renewal within 24-hours prior to the end of the current period.
You may cancel automatic renewal and manage your subscription by selecting Manage App Subscriptions in your iTunes Account settings and selecting the subscription you want to modify. Any unused portion of a free trial period, if offered, will be forfeited when you purchase a Premium subscription, where applicable.