Wordpress Default Login
You can change WordPress default Admin Username and you should definitely do. Why? Security reasons. If someone is trying to break in and enter your blog admin and if your Username is Admin, his half job is done. Do not try this unless you are comfortable with PHPMyAdmin and MySQL. If not, ask someone to do it for you. From WordPress Admin you can not change the Username (though, I wonder why?). First for precaution measures, get Back up of your whole database before doing it.