Only a SuperAdmin can edit another admin's username. So, create another account and promote it to admin, and then make it the SuperAdmin.
To do that, you will need to edit the config.asp file and change this line:
Const intAdminMemberID = 1
You will need to replace the value 1 by the id of the member you wish to be the new SuperAdmin (if you view his profile from the member list, the ID is the last number in the URL). Once you do this, the SuperAdmin can edit the old admin member profile and change his username.