If you're certain that you wish to cancel, then your options are explained here. Cancelling by phone should be quick but is often reported as a miserable endurance trial - try that first, but if there's any difficulties you may wish to cancel by post (use recorded delivery).
Chances are the self-install kit (mainly a VM hub and cables) has already been despatched, and in that case you'll need to wait for a return equipment label, and drop it off at the nearest collection service.