It's best to perform the following change(s) with a LAN cable connected device, as changing the WiFi access password(s) while using a WiFi connected device may result in that WiFi connected device being kicked off of the system, though reconnecting with the new password should get you back on.
To change your WiFi access password(s), go to this page, use admin for the password, and then click LOGIN. On the right you can see the password for the 2.4GHz WiFi access. Use the tabs above to switch to the other band(s) (2.4, 2.4 Guest, 5 and 5 Guest). When you change a password, click Save Settings. You can change one or both (2.4GHz and 5GHz). Make sure to write down your new password(s), of course.
This PDF also contains information about changing the WiFi access password(s), as well as the password to access the WiFi settings (currently "admin") in the modem.
Hope this helps. 