Once the admin user logged into the system, on the left top of the screen admin user will find a list of the menu from that Click on Administration TAB
On the left panel, the admin user will find a list of the folders hierarchy select user administration under that by clicking the user list admin user will get the list of users in the system.
Click Edit icon 
to edit the user details for modifying the existing
Then a pop-up dialog box window will appear, you can modify the details of the particular user and click on save button.