Upgrading is generally a top-down process starting with the Virtual Center Server, then hosts, then VMs. This ensures that everything is properly supported during the upgrade process.
- Make sure vSphere plug-ins are supported in vSphere 5
- Upgrade or remove them if necessary
- Confirm the environment meets the system requirements
- Upgrade vCenter Server
- Upgrade VMware Update Manager
- Upgrade ESXi hosts
- Reconfigure Licensing
- Upgrade VMs
Note that this process will differ somewhat if using VMware View.