You will have to log in to your router, and change the SSID, which will change the network name your WiFi clients see. If you want to change the network key, you should be able to do it on the same config page in the router as you change the SSID on. You will have to re-enter the network key on all your clients. You could keep the same key if you wanted to, but if the intent is to change things because of some security issue, changing the key would be recommended.
The answer to the first question in this message is important in deciding how you proceed.....
Did you purchase the router, or was it supplied by Comcast as part of their Home Networking package?
If it is your own purchase, and you don't have a username and password to access the router, you will have to perform a factory reset, then reconfigure it. Post back for more details if you need them
If you have the Home Networking package, the tech that did the install should have given you a username and password, as they should have been changed from the defaults when they set it up. If they did not leave you a username and password, you should really contact them via phone for support, as you are payiing them to support the Home Networking package.