You require Drupal 8 to install CMS Content Sync. Checkout our Technical Requirements for more information. Make sure your Drupal core and all contrib modules are up-to-date before you continue.
Install the latest module version from Drupal.org by using composer:
composer require drupal/cms_content_sync:~2.2
Composer The module has to be downloaded with Composer so that all dependencies are resolved correctly. If the module isn't installed with Composer, you may run into difficulties due to missing modules, missing libraries or version mismatches.
Enable the module for all sites either with the "Extend" functionality in the Drupal admin UI or by using Drush:
drush @sites en cms_content_sync
Drush 9 The Drush alias "@sites" has been removed from Drush 9 (https://github.com/drush-ops/drush/issues/3019#issuecomment-334503217). Instead of using "@sites", the site builders will have to enable the module on all sites separately.