Drupal. Cycling. Upgrading. This page explains in detail how to upgrade Moodle.
For a summary of the process, see Upgrade overview. Check the requirements Check that your server meets all requirements for 2.9 in Administration > Site administration > Server > Environment. Please note that if you use MySQL you must upgrade the engine to InnoDB since it has become a requirement in Moodle 2.9. See Migration from MyISAM to InnoDB.