I was wondering the Same ie How to Change the WebAdmin Password as i was unable to Access the Advanced Functions even if with SSH i had no problem.
Here is how to do:
Open a SSH Session and type the following commands as a root
[root@YourNAS ~]# export NAS_NBIN=/usr/www/nbin
[root@YourNAS ~]# cd /usr/www/nbin
[root@YourNAS nbin]# ./setAdminUser.sh username password
Wish it helps…