Currently using Fontawesome v4.7 but there is an icon (fa-fire) which has been updated and is included within Fontawesome 5 package. Is it possible to update the css file or the webfonts files without downloading the whole v5 package and removing/adding files?
I fear this might create more trouble than benefit. The icons are now split over several font-files, so you can not just exchange the font-files and edit the CSS for just that icon etc.
But if you really want to use the new icon (and only that one icon), you could use the SVG-Sprite for that. Will still require to change your code (you won't be able to access it as fa-fire), but hey - you'd be able to use a FA5-Icon on a FA4-Site!