Upgrading Drupal version 5.x to 6.x
Upgrading from Drupal 5.x to 6.x
This screencast should help you to quickly upgrade from Drupal 5.x to 6.x
Thanks to Gregg Knaddison for creating the above screencast.
You may find that some of your modules will not work any more after an upgrade. To resolve this please read the next following article on Porting Drupal 5 modules.

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Content Migation
How can i migrate the site content and forum from drupal 5 to 6?
You don't need to migrate
You don't need to migrate your site contents from Drupal 5 to 6. Follow the video screencast on upgrading. At the final stage after you have copied the new files over to your existing installed site, you run the update.php script which should be URL: http://{my_domain_name.com}/update.php.
Replace {my_domain_name} with the name of your site. The update.php script should take care of upgrading the DB which stores all your site contents.