Select a user from the People page to load that user's overview, then select the Edit button on the right side of the page. This will load the user's profile. Alternatively, select the pencil icon alongside a user's name on the People page. This will launch the same profile but in a separate tab.

At the top of the page you'll see the user's name. This is an actionable field. In other words, clicking the name will allow you to change it.

Once you've made your changes, select Save. Your updates will now be reflected throughout the system.