While the process is simple, I still advise caution in following the steps as written – in other words, if in doubt save a backup first

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
(Ex: d60702954)

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!

Related Queries:
How to change my WordPress login default name?
Change Admin login to something else in WordPress?