How can I change a User's email?
A User's email cannot be changed, per se, but you can add a new user (new email address) and remove the old user/address.
Click Organization -> Users -> Add User
An email link will be sent to the new user address to set their password.
See Adding Users for more details.
If you want to change your own email address, then you can add the new email as a user and then once that account is activated sign in and remove the old user.