1. To change a user's email for login, you can simply add a new user by going to the "manage users" tab and click "add user." Proceed to fill in the new user's details including their new email. Please note you can delete the old user profile which contains the previous email but this is not a necessity.
2. You may also have the actual user change their own email for login by having them click on "my account" menu at the top right of the app and thereafter proceed to edit their profile information by changing their email.