VMware provides several ways to upgrade ESXi version 5.5.x and version 6.0.x hosts to ESXi 6.5.
The details and level of support for an upgrade to ESXi 6.5 depend on the host to be upgraded and the upgrade method that you use. Verify support for the upgrade path from your current version of ESXi to the version to which you are upgrading. See VMware Product Interoperability Matrixes at http://www.vmware.com/resources/compatibility/sim/interop_matrix.php.
You can upgrade a ESXi 5.5.x host or 6.0.x host, asynchronously released driver or other third-party customizations, interactive upgrade from CD or DVD, scripted upgrade, or upgrade with vSphere Update Manager. When you upgrade an ESXi 5.5.x host or 6.0.x host that has custom VIBs to version 6.5, the custom VIBs are migrated. See Upgrading Hosts That Have Third-Party Custom VIBs.
High level steps for upgrading ESXi:
Verify that your system meets the upgrade requirements. See ESXi Requirements.
Prepare your environment before upgrading. See Before Upgrading ESXi Hosts.
Determine where you want to locate and boot the ESXi installer. See Media Options for Booting the ESXi Installer. If you are PXE-booting the installer, verify that your network PXE infrastructure is properly set up. See PXE Booting the ESXi Installer.
Upgrade ESXi. See Upgrading ESXi Hosts
After upgrading ESXi hosts, you must reconnect the hosts to the vCenter Server and reapply the licenses. See After You Upgrade ESXi Hosts.
Methods supported for direct upgrade to ESXi 6.5 are:
Use the interactive graphical user interface (GUI) installer from CD, DVD, or USB drive.
Use the esxcli command line interface (CLI).
vSphere Auto Deploy. If the ESXi 5.5.x host was deployed by using vSphere Auto Deploy, you can use vSphere Auto Deploy to reprovision the host with a 6.5 image.
vSphere Update Manager.
Graphical User Interface (GUI) Installer
Upgrade interactively by using an ESXi installer ISO image on CD/DVD or USB flash drive. You can run the ESXi 6.5 installer from a CD/DVD or USB flash drive to do an interactive upgrade. 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 ESXi 5.0.x,ESXi 5.1.x, or ESXi 5.5.x installation, the installer upgrades the host to 6.5. The installer also gives you the option to migrate some existing host settings and configuration files and to preserve the existing VMFS datastore. See Upgrade Hosts Interactively.
Perform a Scripted Upgrade
You can upgrade hosts from ESXi 5.5.x and ESXi 6.0.x to ESXi 6.5 by running 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 specifying a preboot execution environment (PXE) for the installer. You can also call a script from an interactive installation. See Installing or Upgrading Hosts by Using a Script.
esxcli Command Line Interface
You can use the esxcli command-line utility for ESXi to upgrade ESXi 5.5.x hosts or ESXi 6.0.x hosts to ESXi 6.5 hosts. See Upgrading Hosts by Using esxcli Commands.
vSphere Auto Deploy
After an ESXi 5.5.x or ESXi host is deployed with vSphere Auto Deploy, you can use vSphere Auto Deploy to reprovision the host and reboot it with a new image profile. This profile contains an ESXi upgrade or patch, a host configuration profile, and optionally, third-party drivers or management agents that are provided by VMware partners. You can build custom images by using vSphere ESXi Image Builder CLI. See Using vSphere Auto Deploy to Reprovision Hosts.
vSphere Update Manager
vSphere Update Manager is software for upgrading, migrating, updating, and patching clustered hosts, virtual machines, and guest operating systems. vSphere Update Manager orchestrates host and virtual machine upgrades. If your site uses vCenter Server, VMware recommends that you use vSphere Update Manager. For instructions about performing an orchestrated virtual machine upgrade, see the Installing and Administering VMware vSphere Update Manager documentation.
The esxupdate and vihostupdate utilities are not supported for ESXi 6.5 upgrades.