If you don't remember the wifi password it'll probably be necessary to reset it in the router and reconfigure all of your wireless devices with the new password. You can find out more about how to configure wireless settings here:
http://www.arrisi.com/modems/datasheet/SBG6580/SBG6580_UserGuide.pdf
http://www.arrisi.com/modems/datasheet/SBG6580/SBG6580_UserGuide.pdf