Please note that before proceeding, it is advisable to back up your Wordpress site.
To change the default user we need to create a new one, give it administrator priviledges and then remove the old one:
- Log into your Wordpress Dashboard
- Go to Users -> Add New
- After filling in the details, select the Administrator role for your new user and add the new user.
- Log out of your dashboard
- Log in again, but with the user you have just created
- Go to Users, find your old admin account and delete it
- When being asked about what to do with the admin's post, just attribute them to the new user created.
- Now you have a new username, that took over all of the previous admin posts and that will be another security meansure for the website.