If a member's email address needs to be updated, there are 3 options for updating the email address that will properly update the email address in BOTH the membership site and Infusionsoft:
The member can always change their own email address in the membership site (they should be able to change it on the Profile page). When the profile is updated in Reservoir, this will also update the email address (and other information that was changed) in Infusionsoft.
You (an administrator) can update the email address in the Reservoir membership site.
You (an administrator) can update the email address in Infusionsoft. However, when you do this, note that you will also need to then update the email address in the membership site admin area (see section above).
IMPORTANT: Changing the email address in Infusionsoft does NOT update the password in the membership system.