Navigate to Client Area and then click on 'Services'.
Then click on the service you wish to request cancellation for.
Then below 'actions ' to the right you will see a button that is labelled 'Request Cancellation', click it.
Once you have clicked 'Request Cancellation' you will be directed to a page that looks like the following below. You will need to explain reason for cancellation and then confirm you do not wish to renew the domain again if you do not want it.