https://ostraining.com/blog/wordpress/change-usernames/

• Go to Plugins > Add New
• Search for Username Changer
• Click on Install Now

• Go to Users / All Users
• Select the User you wish to edit  (I chose testuser)
• Click Edit (text link underneath the username)
• Click Change Username 
  
• Type the new username (testuser2) and SAVE
  You can double check the name change by going back into Users / All Users.  You will find the new username in your list. (See image below).

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.

That’s all there is to it. You’re done. Congrats!