You can change plans any time. When you upgrade or downgrade plans, your next invoice will be prorated based on the usage of your current plan at the time you switched.
Here's how to change plans:
Navigate to the Accounts Selector at the top right corner of the page
Click Account and select the Billing tab
Under Your Current Plan, click on Change Plan
Choose one of the plans in the selection and you will be given a breakdown of the plan difference which is going to be prorated on your next bill.
To confirm that change, click on Change Subscription.
Your new subscription will begin immediately. Any previous payments you made will be credited toward your new plan. The amount you owe is the difference in price between your old plan and your new plan for the remainder of the billing period.