To edit a user's details on a portal, you must have either the Administrator or Content Administrator role. For Content Administrators to get this access, the Administrator must first give them permission (this is managed via Portal Settings > Configuration).
Click on your name (top right corner) and select 'Administer'.
Select 'Users' and click on the user name you wish to edit.
Select 'Edit' from the drop down next to their name.
Click into the Email or Name fields to edit the user's details and then click 'Save'.
Note: You can only change the email address of a user if it is associated with your company domain and you need to be the Administrator across all portals they access to do that here.
The user will receive a notification via email on their original address, advising that their email address has been changed.