Upgrading Drush 6 to Drush 9 on Centos 5 server
drush dl drush downloads the obsolete version of Drush from drupal.org. Drush was never intended to be used to download Drush via the pm-download command, even when it was hosted on drupal.org; the fact that this works is more of an anomaly or coincidence than design.

To update to a newer Drush, follow the usual instructions in the Drush documentation.  Since you already have Drush in usr/bin/drush, use this location instead of the location stipulated in the docs (usr/local/bin/drush).

wget http://files.drush.org/drush.phar
chmod +x drush.phar
sudo mv drush.phar /usr/bin/drush


The Drush at files.drush.org/drush.phar is always the most recent stable version of Drush. To download a specific version, visit https://github.com/drush-ops/drush/releases. There is a phar available with every release of Drush from version 8 and later.  You can, of course, replace your /usr/bin/drush at any time that you need to upgrade.