Level 88
You can't just upgrade to Font-Awesome 5 because icon names might have changed and the update also requires you to make some changes to the HTML!
So if you really want this, you can do this pretty easily by just loading in the new font-awesome CDN for example and use the correct classes in your icons and you shpuld be good to go!
Documentation: https://fontawesome.com/how-to-use/on-the-web/setup/upgrading-from-version-4