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. Also see https://www.drupal.org/docs/8/update/update-procedure-in-drupal-8

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

cd into /srv/www/vhosts/drupal8_stock/drupal-new/web

run ../vendor/drush/drush/drush updb