Attention: You must back up all of the files and database before upgrade Drupal core
- Tune on maintenance mode on your website, go to admin -> configuration -> DEVELOPMENT -> Maintenance mode
- 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'.
- /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)
- Change the setting of $settings['update_free_access'] to "TRUE" on sites/default/settings.php.
- Browse "update.php" via any browser and finish the upgrade process.
- Change back $settings['update_free_access'] to "FALSE"
- Please check whether the site is working without any errors before tune off the maintenance mode.
- That's all.
Another easy way to update Drupal core is using the web tools: Drupal 7 / Drupal 8 Core Updater