Do not try this unless you are comfortable with PHPMyAdmin and making changes to MySQL. If not, ask someone to do it for you.
First for precaution measures, get Back up of your whole database.
Why Changing WordPress Admin User Name?
One common security vulnerability in WordPress is User Enumeration, in which an attacker enumerates a valid WordPress user account in order to launch brute force (password guessing) attacks. In order to help discourage this type of attack, you should change your admin username to something harder to guess.
To change your admin username:
Open your web host’s MySQL administration tool (probably phpmyadmin) and browse to your WordPress database.
Locate the users table, in which you will find a user_login column.
One of the rows will have admin in the field.
Change this to something else by editing that row.
