Contrary to what is written on your wordpress admin panel, that username cannot be changed, you can actually change it via PhpMyAdmin.
NOTE: This only applies to website admins.
- Login to your CPanel.
- Go to your PhpMyAdmin.
- Choose the right database (if you have several).
- On the left side, look for wp_users
- The first user, of course, is the administrator of the account. Just click on the pencil icon to change your username name (this is the one you are using to login).
- Click Save.
DON’T FORGET: USE THE NEW USERNAME the next time you login.