In this case you need to perform hard reset on the modem since there is no option is to reset login credentials so after resetting modem you can use the default login credentials to access the modem login page. 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/G34-G36-Factory-Reset

http://arris.force.com/consumers/articles/General_FAQs/G34-G36-Web-Manager-Access

Got it figured out. Thank you.