The only way to change a users password is through the NBX Web interface, called Netset.. for this you will need the administrators password. Type in the IP address of the NBX in a browser, then type username:administrator, then the password.. the default password is 0000. so you might try that .. maybe it was never changed.
Then go toSystem Maintenance/Password Adminstration. You will
see a drop down menu, go to User Password and click on "GO". This will bring up all the users.. click on the select box for the users whose passwords you want to change, then click the box "Reset Password". What this does is set the password so it is the same as the extension number of the user.