An employee might be able to send you a return box.
I've asked a Comcast employee to help you. You should expect a reply in this thread.
Hello spoore7, the information that (nerdburg) shared is correct. if you do not wish to use this modem you can take it into a local Comcast service center or a UPS service center. Also the packing the modem came in should have return labels you can mail the equipment back. Please let me know if you have any additional questions or concerns.