Changing your network settings, especially on a per-device basis, isn't easily possible unless you have a newer router - like the Google Wifi. For Xfinity customers using one of the standard Wireless Gateways, you could change basic settings from the Xfinity Account app, but not much else. Now Comcast has released a new app, xFi, that gives customers far greater control over their home network.
The new app relies on some of Comcast's newer modems - namely the Wireless Gateway 3 (excluding TC8717) and Advanced Gateway. If you're not sure if you have the right device, check this support page. If you have an older model installed, you can schedule a technician to install the new router or install it yourself.
Once you're rocking a compatible router, just grab the xFi app (linked below) from the Play Store and login with your Xfinity account. The app allows you to see network activity, change the Wi-Fi network settings, and view all the devices on your network.
One of the more advanced features is creating profiles. You can assign devices to profiles (representing family members, for instance) to get a better sense of your home network activity. If Comcast is enforcing the stupid 1TB cap on your home, this can probably help you figure out who is watching too much 4K Netflix.
If you want to try it out, you can download it for free from the Play Store below. As previously mentioned, this requires a compatible Xfinity Wireless Gateway to work properly. In-depth details about the new functionality can be found here.