To unsubscribe from additional disk space 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 Additional Storage:

You will be redirected to the Subscription Purchase page:

Click the "Unsubscribe" button from the disk space you purchased earlier.

To confirm, click "Unsubscribe".

You will be redirected to the 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".