If you are unable to login to the surfboard central app using your own login credentials there is no option to reset the login credentials on the app so the only option is to uninstall the app and  perform hard reset on the modem then install the app again after connecting device to the modem default WIFI network.  But after resetting the modem will go to default setup any custom settings will be removed so use the default network name and password to connect to your network.

http://arris.force.com/consumers/articles/General_FAQs/SBG10-Factory-Reset