For upgrades that occur in the middle of your billing cycle, you will be billed a proportion of your monthly fee, equal to the percentage of time left in your monthly billing cycle.
You will also receive a refund for your original plan in an amount equal to the percentage of the month remaining at the time of upgrade. Any overages incurred during the month will be charged at the rate associated with overages on the active subscription at the time the overage event occurred.
To update your subscription, access your Account Management Page. Select the product you're inquiring about from the Products tab. Select Manage Plan, then View Billing Reports.