You can create the new password like this:
htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin
(take care of the path to your *htpasswd.users *file)
If you would like to change it, hm, I'm not sure how to do that. Maybe it will let you create a new password and a new htpasswd.users file if you're root. Try it. Of course, backup your htpasswd.users file before you change anything so you could get it back if something goes wrong.
If that doesn't work, add a new user (just replace nagiosadmin username with the new one) and then try to delete the old one if you don't need it.
Check the possibilities with running in terminal: