On March 12, 2015, VMware released vSphere 6 for General Availability. I thought it would be a good time to recap, and pretty up, my older upgrade post. The previous post was based on vSphere 5.5 and the specifics of the software upgrades have changed, but the general order has not.
Read all of the Hardware Compatibility List, Interoperability Matrix, and other similar documents for all your components. Make sure all hardware is supported with ESXi 6 and that all software solutions support both ESXi and vCenter 6. Repeat with all other vSphere suite components such as SRM and the vRealize products. Contact vendors of incompatible solutions and find out when their v6 support is expected. If anything in your list does NOT support the latest version, make a decision on whether to remove or replace the components or to halt the upgrade until they are available.