You can,t edit your User name but can create a new user as Administrator by going to wp-admin > Users > create new user, And give the role of administrator as in the image.
Then log out your old user and log in with the new user.
After login you can change the password by going to Edit Profile.
If you want to delete old user, delete it while deleting it will be asked to do you want to add your all posts in your new user so its depend on you, you want to add or not.
Hope this Article will help you.