The management of the Cisco Wireless LAN Controller (WLC) is done via https interface.
However, at least on software 18.104.22.168, you can't change the administrator password using the web interface.
To do it, you'll need to use the CLI (SSH) interface. SSH to the WLC, then type the command:
config mgmtuser password Administrator NEWPASSWORD
Just type this on the CLI, then, come back to the Web Interface, test it, and save the configuration clicking on the "Save Configuration" link on the top right corner of the webpage.
That's all folks!