When you have connected to the server via VNC please find on the desktop the"NT password" icon. Click on it, when the program has started click on the "Re(open)" button. Now find the root user, click on it, then if your password has expired click on the "Unlock" buttton and then "Save changes", if you want to change the password click on "Change password". In the next window you must type your new password twice. Bear in mind that the password should have both upper and lower case and at least one digit. After entering this you need to click on Save changes.
When you have changed the password you must switch the server into the normal boot in "Netboot", then change it into the HD mode. After that restart the server, when the server comes online you can use your new password.