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
  * run ../vendor/drush/drush/drush cr

For more info on drush, see https://drushcommands.com/