If you have an existing WordPress site and you want to change your username, you can do this using below mentioned steps.
Step 1: Login to your cPanel and click on the phpMyAdmin icon under Databases section.
Step 2: Locate your WordPress database from the list on the left.
If you're not sure which one it is, you can check the wp-config.php file in the root folder of your WordPress installation for the line below:
define ('DB_NAME', 'user_wp448');
Step 3: Once you select your WordPress database, you will see a list of tables. Choose the _users table from the left menu.
Step 4: You will see a list of all the registered users in your site. Locate the admin username and click on the Edit button on that line.
Step 5: Now, you will see all the fields for this username. Locate the user_login field and replace the username admin to your new preferred WordPress login name and press the Go button at the bottom of the page.
Now you can login with new WordPress username.