here’s the guide on how to change it. 

https://arris.my.salesforce-sites.com/consumers/articles/General_FAQs/G18-G20-Changing-the-Wi-Fi-Name-and-Password/

You can change the network credentials both ways, 

The first option is through the SURFboard Central app. By setting up the app, you can easily change the Wi-Fi name and password.

Please find the link below, where you can follow the steps to set up the SURFboard Central app and update the Wi-Fi name and password.

http://arris.force.com/consumers/articles/General_FAQs/SURFboard-Central-Wi-Fi-Cable-Modem-Setup

http://arris.force.com/consumers/articles/General_FAQs/SURFboard-Central-Changing-the-Wi-Fi-Name-Password

The other option is as ​@plemans suggested through the web-based interface (GUI), where you can also change the Wi-Fi name and password.

http://arris.force.com/consumers/articles/General_FAQs/G18-G20-Changing-the-Wi-Fi-Name-and-Password