Well, the "766" number is the number of new RSS items. That News bookmark contains RSS feeds from several sites, and the number in parentheses after the bookmark name in the bar or menu tells you how many new things have appeared.
I'm not actually sure I understand your question, otherwise; in Safari, you edit the bookmarks by clicking the little book icon on the far left of the bookmark bar, or by selecting "Show all bookmarks" from the Bookmark menu. From there you can edit or delete items in the bar, menu, or even folders that appear in neither, if you're a bookmark fiend. For items in the bar, you can also just drag them off and they will disappear in a puff.
If I'm reading right and you're poking around in your Library folder, you're doing it wrong--the end user was never intended to manually touch ANYTHING in that folder, and they've hidden it by default in Lion to discourage doing so. Use the app itself to edit things, in this case Safari.