A coach can change an athlete's email address in the web portal, if the athlete has NOT yet finished setting up their account (created a password, etc). To do this, go to the Athlete list, click on the athlete's name to see their details, and go to their Profile tab.
If the athlete has already set up their account, then the athlete will need to change their email address themselves. They can do this by going into the web portal, clicking on their name in the upper right corner, and selecting My Profile