Use the following steps to change the admin username and define an advanced username:

1. Log in to phpMyAdmin as a root user.
2. Click the database for the WordPress installation from the left-hand menu.
3. Click the wp_users table on the right section of the screen. Depending on the prefix used in your site, the table should be {prefix}_users.
4. Click the Edit link of the admin user as shown in the following screenshot:

5. Type a different username for the user_login value.
6. Click the Go button to save the data.

Now, the admin username will be changed and you can use the new username to log in to the site.