Click on the "domain names" link in the menu on the left. Via the "search" option, you can search for the domain you want to cancel. Click on the name of the domain name you want to cancel, and a new window will open.
On the right side under actions, you can turn off automatic renewal for the domain name. If you choose this option, the domain name will expire on the expiration date.
You can also choose to delete the domain name immediately by clicking on delete. Then keep in mind that the domain name is immediately off the air.