There is no option to reset the password alone on the modem GUI.
If the default password is not working you can try entering word password in the password field then login.
The Cable Operators (MSOs) have complete control of what cable modem/gateway hardware they will allow installed and the firmware it uses, this is because they own the coaxial cable that is connected to the ARRIS (SB8200), because of that the Firmware updates are pushed by them, and any update needed as well. Any associated firmware updates are automatically deployed by the cable operator. This is not an ARRIS only implementation, but the required implementation of any cable device manufacturer. Even for the device owned by the customers only ISP send updated since the device is activated and receiving signal from the ISP.
On the modem interface you can only look on to the modem signal since this is just a cable modem.
http://arris.force.com/consumers/articles/General_FAQs/SB8200-Web-Manager-Access