How to upgrade / update core on Drupal 7 and Drupal 8

   Attention: You must back up all of the files and database before upgrade Drupal core

  1. Tune on maintenance mode on your website, go to admin -> configuration -> DEVELOPMENT -> Maintenance mode
  2. Download the latest version of  Drupal 8 (https://www.drupal.org/8) and unzip the file.
    -Rename the folder to 'public_html', copy and replace following files and folders from your source to the 'public_html'.
    • /sites/default/settings.php
    • /sites/default/files
    • /core/themes (Drupal8 only), /sites/all/themes (Drupal 7 only)
    • /modules (Drupal8 only), /sites/all/modules (Drupal 7 only)
    • /sites/all/libraries (Drupal 7 only)
       
  3. Change the setting of $settings['update_free_access'] to "TRUE" on sites/default/settings.php.
  4. Browse "update.php" via any browser and finish the upgrade process.
  5. Change back $settings['update_free_access'] to "FALSE"
  6. Please check whether the site is working without any errors before tune off the maintenance mode.
  7. That's all.

Another easy way to update Drupal core is using the web tools: Drupal 7 / Drupal 8 Core Updater