Type the IP address into a web browser - Safari, Chrome, Firefox etc and it will prompt you to enter a username and password, which should also be in the manual.
Once you've logged in you will be able to look through the menus to find where to change the password. It'll be under Security if it's the Admin password you want to change at a guess and under Wireless if it's the encryption key you want to change.
It normally is 10.0.0.1 or 10.0.1.1 or 192.168.0.1 or 192.168.1.1, but it can also be found on the bottom of the device or in System Preferences > Network > AirPort or Ethernet > Advanced > TCP/IP > Router.
1. Open a browser window.
2. Type in the IP address for the modem 192.168.0.1.
3. Type in the default Username; admin.
4. Type in the default Password; motorola.
The manual should list the default login and password. There is probably a reset button on it where you can reset it back the the factory defaults.