You can change your account plan at any time. To do so, log into your account as a User with Admin rights, click on the My Profile icon, select 'Billing' from the list of options presented, and then click 'Form Plans' along the left side of the page that appears.
You'll be given a list of the available Plan options for your account along with the features that come with each and the pricing for that Plan. You can click the arrows in the blue columns to the left and right of the plans to scroll through all of our options available.
You can click the Upgrade/Downgrade Plan button below the Plan you'd like to be moved onto. When you downgrade/upgrade, you will receive a prorated credit towards the new plan level for the remainder of the month/year that you already paid for.