There's no on-line way to do so. 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. Not all shops do that, though. 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 marnadavis. If you still need support with cancelling your services, I can help. Please reach out to me via private message and include your full name, service address, and account number so I can assist you.