VMware documentation on the upgrade can be found here.
The upgrade should be done according to VMware Migration Path in four steps:
- vCenter Server
- Virtual Machines (VMtools and Hardware version)
- VMFS datastores
This part shows how to upgrade a standalone host with ESXi installation iso.
- Download the iso from vmware.com, burn it and insert to the server.
- Shut down all your VMs and boot the server from the CD.
Booting to ESXi 5.1 installation / upgrade CD
- After the files are loaded, the installer will find VMFS partition. Press Enter to continue.
VMFS partition is found
- You will be presented with 3 options. Choose „Upgrade ESXi, preserver VMFS datastore”. Press Enter and F11 to confirm.
Choosing an upgrade or a fresh installation
Confirm the upgrade
- Have a cup of tea / coffee while the server is being upgraded.
- When finished, you will get a confirmation message. Press Enter to reboot.
Upgrade is completed
- Enjoy upgraded ESXi server. Start you virtual machines and verify if everything’s fine.
ESXi reports version 5.1