Comcast doesn't allow on line cancellations. You can call, but the most effective way is to carry Comcast's equipment to the nearest local store and turn it back in to them. Be sure to get a receipt, and keep it forever. Also consider scanning or copying it; my shop uses thermal paper, which fades to a blank piece of paper in a week or so.
You can also take your equipment with you to a UPS Store and an associate will pack and ship your equipment.
Bear in mind you may be under a contract and thus subject to an early termination fee. The official word on early termination fees: http://forums.xfinity.com/t5/Billing/Early-Termination-Fees-ETF/m-p/2895117#M96760
Hello Tparker1. We certainly value your business and don't want to see you go. We may be able to assist with your rates in hopes that you'll change your mind, and remain our customer. I can assist with reviewing your account for lower cost options that may be available. Please send me a private message and include your full name, service address, and account number so I can assist you.