When you install Joomla, you also choose and enter a username for your login account (the critical account of the almighty Super User). Although you can enter any administrator username when installing Joomla, many people don't bother to change the default administrator username, admin. However, this generic username is a security risk—hackers only have to guess your password to access to the site.
If you haven't come up with something different during the installation process, you can change the administrator username later on. This is how you do it:
In the backend of the site, navigate to Users | User Manager.
Select the Super User user record.
In the Edit Profile screen, enter a new Login Name. Be creative!
Click on Save & Close to apply changes.
Log out and log in to the backend with the new username.