This can be done by simply downgrading. Go to your "Billing" page".

In the "Overview" section, click "Change 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 cancellation of subscription.

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