How to change the default admin WordPress login name:
Login to your cpanel
Using your hosts MySQL administration tool (I.E. phpmyadmin), locate and select your WordPress database
From your WordPress database, locate and select the wp_users table (Note: the wp prefix may differ) and then click the browse icon
Next click edit
Under the user_login column, change admin to your preferred WordPress administrative login name and then click go
[click to enlarge]
Now, you should be able to login with your new administrative WordPress login username