Removing admin user name

I'm following the instructions on this tutorial
https://www.inmotionhosting.com/support/edu/wordpress/change-wordpress-admin-username-for-security/

When I go to add a new user the username is already hardwired with the word 'admin' despite me already having 'admin' as an existing user.  I can't override it

I'm going around in circles. 

Any ideas ?