Step 1: Select the user and login.
Step 2: Open ‘server manager’ and go to ‘Tools’ and select ‘Active Directory Users and computers’.
Step 3: Click on the existing username and select the ‘Users’.
Step 4: Right click on the user and select ‘Rename’. Accept the warning message.
Step 5: New dialogue box open, here give the corresponding details and click ‘OK’.
Step 6: Double click on the renamed user and select the ‘Account’ tab and validate the new changes.
Step 7: Log out to apply the changes.