How am I supposed to upgrade a site to latest core version? I tried to update a Drupal 7 site to the latest version. The old Drush update doesn't work and it throws an error about a unknown engine type output format.

I tried to delete the web folder, extract Drupal 8 files, change the database connection settings to use the old database, and call update.php. It doesn't work, and it throws errors about MySQL encoding.

I tried to install Drush 9 to upgrade core, but it keeps complaining about the composer.json file.

I tried to install Drush 8, but it shows more errors.

How am I supposed to update an old Drupal installation to latest core version?
From Drupal documentation :


  Strictly speaking, 'updating' refers to taking your site from one 'minor' version to another, as for example, from Drupal 7 version d7.1, to d7.2; or from Drupal 8 version d8.0.0 to d8.1.3, etc.


Which means upgrade can be done only to minor version.

If you want to change the whole version for example moving from Drupal 7 to Drupal 8 you need to migrate your site, and this normally involved a data migration and restructuring, Also changing and reviewing your custom modules code.

See Upgrading from Drupal 6 or 7 to Drupal 8