Server version is 4.X then
Open DA
Go to user properties and click Rename User link
Write the new name
Select Run Now
If 5.x then:
Open DA
Go to User Management and select Users
Mark the checkbox in the left of the user
From Tools menu select Reassign User
Just put the name of the user you want – Its great! The user does not actually have to exist.
Select Run Now
Check report in /System/sysadmin/reports/userrename – that will have the name of the user and you can see if it has worked.