VMworld 2012: Session VSP1367 – Upgrading vSphere
An in-depth overview of the vSphere upgrade process; includes a detailed overview of the upgrade procedure along with tips and recommendations to ensure a successful upgrade with minimal disruption to your virtual machines. Covers pre-upgrade, upgrade and post-upgrade considerations to include guidance on upgrading VMware Tools and VM compatibility levels.
Source: VMworld 2012: Session VSP1367 – Upgrading vSphere (Youtube).