How to change admin user name?

The first thing I would do is make a backup of your entire site, including the database.

You have a couple options for this. You can create a new user with administrator privileges, update the author on all posts from admin to that user, make sure everything is working correctly, then delete admin.

You can also change this using phpMyAdmin. Log into phpMyAdmin, select Databases, select the wp_users table, find admin under the user_login column and change it to the new name you want, press Enter. You will need to log back in to WordPress after this using your new username and password you had set for admin. Note: this does not automatically update the author of your posts, but you can bulk update them. Let me know if you have any questions.