Hi Desmond,
Do you mean inside Mango, or for SSH?
In Mango, you can navigate to the /users.shtm page when logged in as admin and change anything other than the admin's username. To change the username of admin, either run something like this in the SQL console: UPDATE users SET username='Desmond' WHERE username='admin'; then log out and in or you can create another admin user as desired, then switch to them and either rename the admin user or remove it.
For SSH, the credentials provided have sudo privileges. There are many guides and answers around the internet for renaming Linux a user, but you could consider making a different Linux user, changing the default user's password using passwd and, if your interest is security, restricting SSH login to this possibly less privileged user (although this could prevent you from sudo access over SSH as well).