Overriding FontAwesome in Cassiopeia child template.

gistfile1.txt

First upload your new fontawesome css files to /media/templates/site/cassiopeia_yourchildtemplatename/css
Then upload your new fontawesome webfont files to /media/templates/site/cassiopeia_yourchildtemplatename/webfonts

Then create a joomla.asset.json in your /templates/cassiopeia_yourchildtemplatename/ folder with these contents:
replace ###CHILDNAME### with your childtemplate name (the part after the underscore).

{
  "$schema": "https://developer.joomla.org/schemas/json-schema/web_assets.json",
  "name": "cassiopeia_###CHILDNAME###",
  "version": "4.0.0",
  "description": "This file contains details of the assets used by Cassiopeia, the default Joomla 4 site template.",
  "license": "GPL-2.0-or-later",
  "assets": [
    {
      "name": "fontawesome",
      "type": "style",
      "uri": "all.min.css"
    }
  ]
}

Make sure the uri above has the right filename. it is already automatically looking in media/site/templates/cassiopeia_yourchildtemplatename/css folder.