To change your plan:
1) Log in to your account
2) Click on "My Account"
3) On the Left Side-bar, click on "Plan & Billing"
4) Click on "Change Subscription"
5) There you will see a drop down with an option to switch plan.
If you are not seeing this option on your account, you are most likely not logged in as the administrator, but as a sub-user. Try to log into the dashboard using your administrative account information and you should see the subscription area.