You can pause the campaign and change the copy. The system will just pick up where it left off with that contact. So if they received email 3 they will get email 4 however many days later. Then the system will use the same sequence for the steps, e.g. if email 4 is a 'reply in same thread' email, then it will use the copy from the new email 3 to create the thread.
Just remember to start the campaign again after making your edits.
