How to change the default admin WordPress login name:

Login to your cpanel

phpmyadmin

Using your hosts MySQL administration tool (I.E. phpmyadmin), locate and select your WordPress database

selectdatabase

From your WordPress database, locate and select the wp_users table (Note: the wp prefix may differ) and then click the browse icon

wpuser

Next click edit

edit

Under the user_login column, change admin to your preferred WordPress administrative login name and then click go

changename

[click to enlarge]

Now, you should be able to login with your new administrative WordPress login username