So you'd like to change a username on your site?
Unfortunately, a default installation of WordPress doesn't allow username changes.
When you go to edit a user, the username is greyed out and there's a "Usernames cannot be changed" message beside it.
- In your admin are go to Plugins > Add New.
- Search for "Username Changer".
- Click on "Install Now" for Daniel J F Griffiths' plugin.
- Click "Activate Plugin".
Note, at the time of this writing there's another plugin "WPVN - Username Changer" which has four 5-star reviews. I've opted to use Daniel's instead. It has more reviews (16) and the only two negative reviews had the issues resolved quickly. So it appears to be well supported too.
- Go to Users > Username Changer.
- Select your user from the "Current Username" dropdown.
- Enter your new username.
- Click "Save Changes".
If it's your username that was changed, you'll need to log back into the admin.
That's all there is to it. You're done. Congrats!
Finally, If you won't be using the plugin regularly go ahead and uninstall it. That way, just in case there were to be security issue with it, your site won't be affected.