Update email address on PO workflow notification

On one of the PO's we had wrong email address so the notification is still undelivered. Now the email address has been corrected but since the notification activity has started it is still trying to send it to the old email address. My question is how to update the email address. this email is supposed to go to external party so I tried reassigning it by selecting the TCA parties/email ID but the application throws an error saying it can only be reassinged to employee/users. Let me know if there is any other option.