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