Hi there,
You can get the update package for Joomla 3.8.0 here https://github.com/joomla/joomla-cms/releases/tag/3.8.0
Just download package name Joomla_3.8.0-Stable-Update_Package.zip
After that, go to Components -> Joomla Update -> tab Upload & Update -> Upload the package and click button upload and install
The issue here that your site load plugin with FULL URL PREFIX via joomla administrator like so
http://www.your-domain.com/index.php/en/plugins/system/helix3/assets/css/font-awesome.min.css
it should be
http://www.your-domain.com/plugins/system/helix3/assets/css/font-awesome.min.css
