Is it just the Nagios password (for the web interface) you need, or the root password to the server?
The Nagios password is relatively easy:
[list]1. Locate your Nagios installation on the server
2. Use htpasswd2 -c etc/htpasswd.users nagiosadmin to set a new password for the nagiosadmin user.[/list:u]
(if you do not have htpasswd2, try htpasswd instead)
That should be all there is to it.