Changing the username of the 'owner' and first account (the superadmin) from "admin" to something else, can only be done in the (MySQL) database itself. The username must be changed in 2 db tables:

  1. wp_users

    SELECT * FROM 'wp_users' WHERE ID = 1

    The field user_login originally reads:  admin

    Change it to your preferred username.

  2. wp_sitemeta

    SELECT * FROM 'wp_sitemeta' WHERE meta_key = 'site_admins'

    The meta_value for that key is similar to:

    a:2:{i:0;s:5:"admin";i:1;s:5:"another username";}

    Again, change the admin part in the larger string to your preferred username. Do NOT remove the surrounding quotation marks ""