Is there a way to edit an existing nautilus (file manager) bookmark?

Neal's answer isn't applicable for the current combination of Ubuntu 16.04 and gtk3. The details are stored in ~/.config/gtk-3.0/bookmarks.

Looking in Ubuntu, there doesn't seem to be an option to edit the fancy stuff, just the server and the name used in the menu (there is an "edit bookmark" entry in the bookmark menu, but I'm sure that you found that).

The actual details are stored in a text file ~/.gtk-bookmarks, so it should be reasonably easy to edit the file.

Perhaps you would like to file a bug either for your distro or direct with Gnome.