Have been using the password that came with the router from AT&T and now trying to chTrying to change the password on our Mac. Please help

the mac does not decide the password of the router, the routers configuration does, various routers let one configure their router in different manners, many has a webserver one needs to access using a web browser, if it was me then I would look for the att routers documentation or on atts site as to how / what IP address to type into the router to access it's webserver