Hey,
The fontawesome css file (fontawesome-all.min.css) is located in public/assets/css directory, and the webfont files are inside public/assets/webfonts.
Not sure how FontAwesome Pro is structured but I guess that just replacing those files should be enough.
Keep in mind though that after you replace the files you need to run npm run dev or npm run production to actually merge and minify the CSS files that application is using. If you are not familiar with Laravel Mix and with running those commands, I would recommend you to check Customizing Stylesheets article as well as the Laravel's documentation to learn a bit more about it.
- Milos