To change the plan you are currently subscribed to, go to Account Management.
Click on the Billing tab. Underneath the Plan information is a link to change your plan.
Here, you'll be able to upgrade or downgrade your subscription, and the charge or credit will be automatically prorated.
If you upgrade to a more expensive plan, part of what you've already paid will go toward that fee. Also, if you are downgrading, a portion of what you've already paid will go toward your next month's charge.
You can also cancel your subscription from this page if you need to take a break.