An admin cannot change email id of a member. Only a member can change his/her email id by going to Home->My profile, and click on “Edit Profile”. And click on “change email id” link present in Email address field.
An admin has to inactivate the member with the old email id, by going to Directory->Members, click on “Inactivate User from this house” present in Actions column.
Now, go to Directory->Add Members and add the member into the portal with new email id.