If you do not want to renew your account a the end of the billing cycle, your account will automatically be cancelled after the 30-day grace period. You can simply not pay the current invoice and the account will be automatically suspended and eventually terminated.
If you’d like to cancel early you can always do so in the Client Area by clicking on View Details next to the account you'd like to cancel.
Then in the left sidebar under Actions, select Request Cancellation.
You'll then want to complete the process by confirming the cancelation and requesting to cancel either immediately or at the end of your current billing period.