To change your plan:

1) Log in to your account and click on My Account.

My Account

2) On the Left Side-bar, click on Plan & Billing and select Change Subscription

Plan and billing

5) There you will see a drop down with an option to switch plan

Change subscription 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.