Open a web browser and go to http://192.168.1.254
Enter your AT&T gateway password if prompted
Select Home Network
Select Wi-Fi
Choose the Wi-Fi network band you want to change
Find the Wi-Fi Password field
Enter your new Wi-Fi password
Select Save
Wait for the settings to apply
Reconnect your devices using the new Wi-Fi password
If needed, update the password for both 2.4 GHz and 5 GHz networks