...
To change the Web Users passwords, if you have not integrated Apache with another form of Authentication, it will be using the default method which are users defined in a file managed with htpasswd.
cd /usr/local/nmis8/conf
The default installation location for NMIS8 is /usr/local/nmis8, it may be instainstalled somewhere else, adjust the commands accordingly
cd /usr/local/nmis8/conf
htpasswd users.dat <username>