Could you please let us know if you are trying to change the Wi-Fi credentials or the web interface login password?

If you are trying to change the WI-FI password, you can do so through the surfboard central app and also in the web interface. 

Please refer to the link below, where you can find the steps to change the Wi-Fi password in the Surfboard Central app.

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

Please refer this link below, where you can find the steps to change the WI-FI password in the web interface. 

http://arris.force.com/consumers/articles/General_FAQs/G34-G36-Changing-the-Wi-Fi-Name-and-Password