1.Create a New User
2.Delete The Old One
The easiest way to change your WordPress username is by creating a new user with your desired username and with the administrator user role.
Chose a new email address.
log in with the user account you just created.
Back to the Users section and click on the Delete link below old username
Make sure that you click on ‘Attribute all content to:’ option and then select the new user.
Click on the ‘Confirm Deletion button to delete the old user account.
Don’t forget to add a new email address to Your Gravatar account
That’s all you have successfully changed your WordPress username. If you want you can change the email address of the new user to use the old email address associated with the old username.