D-Link DES-3550: can’t change password

I have a D-Link DES-3550 switch, Firmware: Build 5.00-B28. I want to change admin password, but somehow I am unable to do this:

               DES-3550 Fast Ethernet Switch Command Line Interface

                        Firmware: Build 5.00-B28
      Copyright(C) 2000-2004 D-Link Corporation. All rights reserved.
username:administrator
password:****************************

DES-3550:admin#show account
Command: show account

Current Accounts:
Username         Access Level
---------------  ------------
administrator    Admin

Total Entries : 1

DES-3550:admin#config account administrator
Command: config account administrator

The username does not exist. Enter the name of an existing user.

DES-3550:admin#

Any help would be greatly appreciated!