Which one is your "admin" user? You can use "cpconfig" (for MDS: "mdsconfig") to delete this user and then you can create the "admin" user again and set up the password.
[Expert@mds:0]# mdsconfig
Welcome to Multi-Domain Server Configuration Program
=================================================================
This program will let you re-configure your Multi-Domain Server configuration.
Configuration Options:
----------------------
(1) Leading VIP Interfaces
(2) Licenses
(3) Random Pool
(4) Groups
(5) Certificate's Fingerprint
(6) Administrators
(7) GUI clients
(8) Automatic Start of Multi-Domain Server
(9) P1Shell
(10) Start Multi-Domain Server Password
(11) IPv6 Support for Multi-Domain Server
(12) IPv6 Support for Existing Domain Management Servers
(13) Exit
Enter your choice (1-13): 6
Configuring Administrators...
=============================
Following is a list of the currently defined Administrators
and their Multi-Domain permission levels:
1) user1 Domain Manager
2) user2 Multi-Domain Superuser
3) user3 Domain Manager
4) user4 Domain Manager
5) admin Multi-Domain Superuser
Do you want to add Administrators (y/n) [y] ?
*I used a MDS for example because I am already connected in this. Just an example.