Assuming you are using Composer to maintain your Drupal 8 installation, you can use the following command to update core and all modules (and dependencies).
composer update drupal/* --with-dependencies
If you started you project from "drupal-composer/drupal-project" the command becomes the following.
composer update drupal/* webflo/drupal-core-require-dev --with-dependencies
I recommend reading https://www.drupal.org/docs/8/update/update-core-via-composer and https://www.drupal.org/docs/8/update/update-modules.
Always after updating, don't forget to run database updates and rebuild the cache.
If you started your Composer project from drupal/drupal as the user guide says, you are in trouble, since drupal/core isn't listed as installed. You could try to use Composerize Drupal to fix the problem.
If you started your project from a tar-ball, you also need use Composerize Drupal if you don't plan to do manual updates forever.
Drush 9 doesn't support updating core or modules. It will tell you to use Composer. Anyway, reading the Drush docs is very useful.