config

drush config-import --partial --source=modules/custom/MYMODULE/config/install/

/**
* Add settings to site config.
*/
function MYMODULE_update_9001() {
 $path = \Drupal::service('extension.list.module')->getPath('MYMODULE') .

use Drupal\Core\Config\ConfigFactoryInterface;

...

 $config_factory = \Drupal::configFactory();
 $config = $config_factory->getEditable('system.maintenance');
 $config->set('message', 'test');
 $config->save(TRUE);

/**
* Implements hook_uninstall().
*/
function mymodule_uninstall($is_syncing) {
 \Drupal::configFactory()->getEditable('CONFIG_NAME')->delete();
}