There are a number of reasons as to why you may want to amend an existing User's details, these could include a change of name, Job title or the need to change their permissions.
In order to access this area, when on your site simply click on the Launchpad, followed by clicking on the Cog. Then, from the displayed Apps select 'User Settings', doing so will present you with the list of Users that have access to the site. From here it is a matter of selecting the desired User and amending the relevant details. Toggling between the User's 'Profile' details and 'Permissions' is done via the menu on the left.