Upgrading is the process of moving your site from a previous major version of Drupal to a newer version, for example from Drupal 7 to Drupal 8. This consists of upgrading the codebase to the appropriate version and then migrating the data from your old site into the new one. Drupal 8 core contains two modules to help facilitate this process: Migrate Drupal and Migrate Drupal UI.
Make sure you understand the difference between updating and upgrading. If you already have a Drupal 8 site and want to move to another minor version, see the Minor Version and Security Updates topic.
Should I wait until Drupal 9 is released before upgrading? Because Drupal 9 will be basically Drupal 8 streamlined, trying to skip updating to Drupal 8 will not bring benefits. Skipping Drupal 8 would mean you have less time to execute the upgrade and therefore more risk in your project. Doing the upgrade to Drupal 8 with an eye to Drupal 9 is the best approach. You can already benefit from all the goodness in Drupal 8 then including the easier upgrade paths going forward.
- Upgrade a site from Drupal 6 to Drupal 8
- Upgrade a site from Drupal 7 to Drupal 8
The upgrade process, and underlying Migration API, are still evolving. We don't anticipate them changing dramatically even though they are still marked experimental. Beware of any resources that were written before Drupal 8.1.x was released (April 19, 2016) as the system changed considerably in ways that invalidate many of the older articles.