In the left hand side menu, click Settings.

The details of your current plan is found under the Billing tab. Here you will see how many users are registered in comparison to the maximum users allowed as well as how much storage has been used in relation to your storage capacity. You cannot downgrade to a lower subscription if you currently have more users or storage used than allowed in the lower plan level. Click the Change plan button to select a new plan.

At the Select New Plan dialog, select a new plan and click the Change Plan button.

Upon success, your plan will be reflected with the new changes.