You can easily change a person's email address from Messages in the following simple steps.
While logged into V3, select Messages view (speech bubble icon, brown circle in image below) and then select whichever category the person's message is under. In our example, we selected All Messages (gold box in image below), so we can see everyone.
Select the number with the email you want to change.
The profile of the person would be displayed on the right hand of the screen.
Hovering over the email field would make the Edit option appear (see image below). Click on it.
Input the new email address, and then click Save (see image below).
The email would be updated to the new address on the profile of the person.