just to rule out the really obvious possibility...are you sure the actual css file is not updating? when you view the file as text in your browser (by going to yoursite/filename.css ), are the changes you made absent?
do you know how to hard-refresh? hold the left control (ctrl) key and, still holding ctrl down, press f5.
if you hard-refresh the page your css changes should apply to, do they apply?
is it possible for you to save the css file locally, make changes and save it locally (such as in notepad), and then upload that file to neocities, replacing the old one?