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:
SELECT * FROM 'wp_users' WHERE ID = 1
The field user_login originally reads: admin
Change it to your preferred username.
SELECT * FROM 'wp_sitemeta' WHERE meta_key = 'site_admins'
The meta_value for that key is similar to:
Again, change the admin part in the larger string to your preferred username. Do NOT remove the surrounding quotation marks ""