Is it the admin password or the WiFi password you wish to change? Do you have any information on the bottom of the modem such as an address, username and password like that below?
there should be a sticker on your modem with the default username and password, it also have the web address interface. Access it and goto the setup section it guess. It is a new modem, I might order one from optus and play with it.
The username is optus the password is the same as the default wifi password. It's that simple (I had issues with the password when I got my first one)
I'm not at home so this is from memory.. after login click the big box on the left (system or something??) And then up the top it's in maintenance or username or something like that. It's the top right. When I get home in a few hours I'll give better directions lol