Download Page:
Release Notes:
https://www.vmware.com/support/vcd/doc/rel_notes_vcloud_director_512.html
Also, for ANYONE upgrading from vCloud Director 1.5.x to 5.1.x, please ensure to follow this KB article during the upgrade process:
Title: Detecting and resolving network adapter type mismatches before upgrading vCloud Director to version 5.1.x