There is no option to reset the modem login credentials or forget password so you can login to the modem GUI after resetting the modem completely using default login credentials. 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. After resetting modem connect you device to the modem default WIFI network then follow the steps mentioned on the below article.

http://arris.force.com/consumers/articles/General_FAQs/SVG2482AC-First-Time-Installation