Standard user
If you need to change the email of your own user:
Main menu > Cerebro > My profile...;
Change your email address.
If you're a Universe Administrator and you need to change another user's email:
Main menu > Tools > Administrator...;
Users tab;
Select the user from the list and change his email address to the right.
Email user
Email users can't change their email on their own. Only a Universe Administrator can do that:
Main menu > Tools > Administrator...;
Users tab;
Select the email user from the list and change his email address to the right.