Navigate to Organisation and Users via the Menu.
Click on the Users tab on the left-hand side.
Locate the User by searching for them in the search box or locating them in the list of Users.
Click on their Username. In the administration area, scroll down the page and click on the Update Login Details within the Actions box on the left-hand side.
Type a new Username into the box.
You will be taken to the User Settings tab where the updated Username will appear.
If you also need to amend the User's name, please see the amend a User's name or email address guide.