Upgrading a site from Drupal 6 to 7 using Drush
You could use the Migrate module for this purpose.


  The migrate module provides a flexible framework for migrating content into Drupal from other sources (e.g., when converting a web site from another CMS to Drupal). Out-of-the-box, support for creating core Drupal objects such as nodes, users, files, terms, and comments are included - it can easily be extended for migrating other kinds of content. Content is imported and rolled back using a bundled web interface (Migrate UI module) or included Drush commands (strongly recommended).


For more details you could go through the README.txt of this module.