Upgrading Drupal
This guide has sections covering major version upgrades of Drupal core (for example, Drupal 6 to Drupal 9, or Drupal 9 to Drupal 10.)
There are separate instructions for minor and patch version site updates. (For example, updating from Drupal 9.3.6 to 9.3.8, or 9.2.14 to 9.3.8.)
Preparing your site to upgrade to a newer major version
There are tools available to ensure your Drupal site is compatible with new major releases of Drupal core.