In the top menu, click 'Users'.

If necessary, use the search box to find users.

Click the user options arrow next to the user whose details you would like to edit.

In the drop-down menu, click 'Edit User'. 

You can now use the 'Edit User' pop-up window to edit the user's details. 

When you are finished editing the user's details, click 'Edit User' to save your changes.