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 drivers or management agents provided by VMware partners. You can build custom images by using ESXi Image Builder CLI.
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.