Log in to router at attwifimanager or

user admin

password admin 

(Do not change the login password!  Only the wifi.  If you change the login password and forget it, the router will have to be reset to retrieve the password and all your setting will be erased and YOU WILL HAVE NO INTERNET!)

click on settings

click on Setup the Wireless.  Over to the right you will see the current wi-fi password.  Change it to what you want then scroll to the bottom and click apply.