If you no longer need Premium or Business access, you can easily cancel your subscription from within your account.
Click your name at the top right of any page and then choose Account.
On the next page, click the Billing tab and you will see the option to cancel your subscription. If you have a Business account, you will also see the option to downgrade to Premium.
The last step asks that you provide your reason for discontinuing your subscription.
Please note: If payment has been processed for the month, cancelling your subscription does not refund the charge. You will retain access to your subscription until the end of the billing cycle.