Drupal 8 has been installed using composer and can be managed using the drush commandline tool. See an overview on https://agencychief.com/blog/how-install-drupal-8-composer-then-set-drush for more info.

To update drupal and it's modules do the following:

SSH to the server

cd to /srv/www/vhosts/drupal8_stock/drupal-new/

run composer update --with-dependencies to update drupal and the modules.