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.
Download and Upgrade Information
Archive copies are available for download at https://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.