Please ensure that you only cancel your domain if you no longer require it. If you instead wish to transfer your domain to another provider please follow the instructions in this article.
If you wish to cancel a domain, login to your control panel and select Registered Domains from the home page of your control panel.
You will see a list of your domains. Select the one you wish to cancel and then click the Cancel Domain button. Please note that this will only cancel the domain - any associated web hosting service will need to be cancelled separately - please read this article for more assistance.