How do I update a Snapshot?

I'm using version 15.5.1.  I have one Snapshot named "BaseLine" and from it I created 3 different courses and saved each "C-1", "C-2", "C-3". I've been making changes to the "C-3", but I can't update "C-3". It want's to create a "Snapshot 5" next to "C-3".