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How to Upgrade ESXi 5.1 to 5.5

VMware provides several ways to upgrade ESX/ESXi hosts.
  • vSphere Update Manager
vSphere Update Manager is software for upgrading, migrating, updating, and patching clustered hosts, virtual machines, and guest operating systems.Update Manager orchestrates host and virtual machine upgrades. If your site uses vCenter Server, VMware recommends that you use Update Manager.
  • Upgrade or migrate  interactively using an ESXi installer ISO image on CD/DVD or USB flash drive 
You can run the ESXi 5.5 installer from a CD/DVD or USB flash drive to do an interactive upgrade ormigration. This method is appropriate for deployments with a small number of hosts. The installer works the same as for a fresh installation, but if you select a target disk that already contains an ESX/ESXi 4.x, ESXi 5.0.x, or ESXi 5.1.x installation, the installer upgrades the host to 5.5, and gives you the option to migrate some existing host settings and configuration files, and preserve the existing VMFS datastore.
  • Perform a scripted upgrade
You can upgrade or migrate hosts from version 4.x ESXi and ESX, version 5.0.x ESXi, and version 5.1.x ESXi to ESXi 5.5 by invoking an update script, for an efficient, unattended upgrade. Scripted upgrades provide an efficient way to deploy multiple hosts. You can use a script to upgrade ESXi from a CD, DVD or USB flash drive, or by PXE-booting the installer. You can also call a script from an interactive installation.
  • vSphere Auto Deploy
Auto Deploy is a new feature in vSphere 5.x. After an ESXi 5.x host is deployed with Auto Deploy, you can use Auto Deploy to reprovision the host and reboot it with a new image profile that contains an ESXi upgrade or patch, a host configuration profile, and, optionally, third-party driversor management agents provided by VMware partners. You can build custom images by using ESXi Image Builder CLI.
  • esxcli 
You can use the esxcli command-line utility for ESXi to upgrade or patch ESXi 5.1.x hosts to ESXi 5.5. You cannot use esxcli to upgrade ESX/ESXi 4.x hosts or ESXi 5.0.x hosts to ESXi 5.5. The esxcli command-line utility requires the vSphere CLI.
Also check below guides,

Step By Step Guide How to install VMware ESXi 5.5 within VirtualBox 4.2.18 on Windows 8.1 x64
Step By Step Guide How to install VMware vSphere ESXi 5.5 in VMware Workstation 10

Here we are going to Upgrade or migrate  to ESXi 5.5 interactively using an ESXi installer ISO image on CD/DVD or USB flash drive.
So lets start,
My current version of ESXi is 5.1  as shown below,

Now turn of the ESXi 5.1 server & mount ESXi 5.5 ISO file & power on the ESXI server for upgrade,

Press F11 to accept the EULA and continue.

Select the virtual disk where you want to install, and press Enter to continue.

Select option “Upgrade ESXi, preserve VMFS datastore” as we want to retain old VMFS partition as it has our Virtual machines.

  1. Confirm that the information on the screen is correct, then press F11 to start the installation.

When the upgrade is complete, Press Enter to reboot.
Now you can see ESXi 5.1 is upgrade to 5.5 without any data loss…