To unsubscribe from Pro and Premium plans you would go to your Meetings page and click the "Billing" button:

You will be redirected to My Subscriptions. Click the "Change plan" button near Subscription Plan:

You will be redirected to the Subscription Purchase page:

Click "Unsubscribe", to confirm this action click "Unsubscribe" in the pop-up window.

You will be redirected to the page "My Subscriptions", which will contain information about the cancellation of the subscription.

Also, you can keep the subscription by clicking on the button "Keep this subscription".
 
Please note:  If you have purchased additional Disk space and Live streaming feature, you need to unsubscribe from these subscriptions separately.