Note: The working version number for version 2.0 was 1.6.
On Monday, December 26, 2005, WordPress version 2.0 was released to the public. Specific information on downloading and the new features of 2.0 can be found at WordPress Development 2.0. For information on the changes and improvements in this new version, see Changelog for 2.0.
Download and Upgrade Information
Archive copies are available for download at http://wordpress.org/download/release-archive/
WordPress Forum Discussion with how-to steps and troubleshooting regarding the update:
There were a number of changes from 1.5.2 to 2.0. Some of the key modifications included a completely new administration interface, a persistent cache, and new user role system.