Once you log in as root or a superuser, just run passwd. This will change the password of the account you are currently logged in as.
passwd user1 will change the password for the user1 account if one exists.
To see what other user accounts are created. Look in the /etc/passwd file. This is where all account info is kept. You can look in the file by using cat or vi. so cat /etc/passwd or vi /etc/passwd.
This will show all accounts however. So system accounts will be shown too. If you want it really easy, jump into your gui and run redhat-config-users this will show you what user accounts are around.