Read through this thread and this thread.. It sounds like you need to update /etc/apt/sources.list. I’m not sure if there’s a simpler way for you to do that besides running an editor (e.g., vi) from a terminal prompt in Cockpit, which might feel a bit hairy if you haven’t done something like that before. In that second thread, you’ll find the suggestion “For any other non-techies following a long, I found this article helpful in working out how to edit the file https://www.howtogeek.com/102468/a-beginners-guide-to-editing-text-files-with-vi/”.