A correct answer from Clem, but hard for a newbie to understand I think.
I'd advice you to use gnome-control-center.
Editing sources.list may be a bit "to much".
Open a terminal and paste gnome-control-center (of course enter
and it opens up
The updates are to some extent already for Cassandra even if she is not released yet.
Oh and don't close the terminal until you're done because that would close what ever you opened.
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