Actually, the latest version of macOS is Big Sur, soon to be replaced by Monterey. You're currently running the latest and greatest macOS that supports your 2011 iMac. You can't move to a more recent version of the operating system without getting new hardware...
Thanks Ron. I found the same instructions, but unfortunately none of that works. I can select all bookmarks or a range of bookmarks, but hitting the delete key does nothing, and as I mentioned, double-tapping to bring up a context menu removes all of my selections. Weird.
Ron, since you're a seasoned Safari user, do you know how to delete ALL macOS Safari bookmarks at once? When I choose Edit Bookmarks, I can shift+click to select everything, but as soon as I double-tap to bring up a context menu my selections go away.