Where do I place my edited class.communication.php file in membership?

I guess my old thread was deemed abandoned since I'm not getting any responses: https://premium.wpmudev.org/forums/topic/can-we-change-the-membership-welcome-email-to-help-with-aweber-integration?replies=5#post-621499

So I'll start with a simple question: I have edits to the class.communication.php file in order to send a copy of a post-purchase email to my aweber list. I think this will help me integrate aweber and membership, but where do I place the edited file?