01- Using your hosts MySQL administration tool (called usually phpmyadmin thru the MySQL menu), locate and select your WordPress database for your blog
This should open up your phpmyadmin window
Now select your database name
02– From your WordPress database, locate and select the WP_USERS TABLE (Note: the wp prefix may differ) and then click the BROWSE icon
03– Locate ADMIN and click the EDIT icon (on left)
04– Under the USER_LOGIN column, change ADMIN to your preferred WordPress administrative login name and then click GO
Thats it, you should now be able to login with your new user name!
How to change my WordPress login default name?
Change Admin login to something else in WordPress?