Navigate to the Menu and click on Organisation and Users.
Click on the Users tab on the left-hand side.
Locate the User by typing their name into the Search Users box and clicking on the magnifying glass or select them from the list.
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.
Click on Submit.
You will be taken to the User Settings tab where you will be able to see confirmation that it has changed.
If you also need to amend the User's name, please see the amend a User's name or email address guide.