Please show the link for that...
The Republican and Democrat parties are private not public entities. They are free to replace candidates any time they choose. It isn’t up to Congress or anyone else to make that decision. So in this case if Hillary had to be replaced, even at this late date, The Democratic National Committee would meet and choose one.
Here is link on the rules and they are about the same for both parties: http://www.gopbriefingroom.com/index.php?topic=222921.0
By the way all you had to do was type the question in a search engine and you could have found this easily like I did.