I recently upgraded FontAwesome on v1.15rc, so here's a little roundup of what I did in case anybody will encounter difficulties.
Download the latest version of FA from their website. You need this package.
Unpack and upload the files to your server at "(nodebb_dir)/public/vendor/fontawesome". You only need the contents of "/webfonts", "/less/_variables.less" and "/less/_icons.less". Don't upload anything else.
Make a copy of your old icons.less and variables.less, then replace them with your new files (remove the underscore).
Run the script provided by NodeMoster. I modified it a little and removed grep:
That is because in version 5 this icon is called fa-var-arrows-alt-v
After a successful build you can start your forum. If you installed a free version of FA you may want to change the font-weight for .fa, otherwise, a lot of icons will be missing. You also need to set vertical-align to some elements (like categories) as the new icons are aligned differently.
//experiment with this, some icons look better with the alignment, some don't