Option 1 – Change username in PHP.  This is from https://wpsecuritylock.com/how-to-hide-wordpress-username/

1. Login to your WordPress Dashboard
2. Go to Users > All Users
3. Mouse over the user you want to change and click “Edit”
4. Fill in “First Name”
5. Fill in “Last Name”
6. Fill in “Nickname” such as First and Last Name (example: John Doe, Queen of Sheba)
7. Change the dropdown for “Display name publicly as” to something other than your username
8. Login to your cPanel
9. Open “phpMyAdmin”
10. Open your site’s database
11. Click “Browse” link next to “wp-users” (you may have changed your table prefix, so look for _users)
12. Change the user_login (username) to something different, but keep the same user_nicename (author url)
13. Or change the user_nicename (author url) and keep the same user_login (username)

Option 2-  Use a Plug in

1. Install the iThemes Security Pro plugin and configure
2. Go to Security > Settings > WordPress Tweaks and enable “Force Unique Nickname”
3. Do Steps 1 through 7 (above) for each user