What command line utility can be used to upgrade an ESXi host?

esxcli is the correct utility because it includes the 'software' namespace, which allows administrators to apply VIBs and upgrade profiles to an ESXi host using commands such as 'esxcli software profile update'. It is the VMware-supported replacement for legacy tools and can be run both locally on the host shell and remotely via the vSphere CLI.

esxupdate was a utility for classic ESX (not ESXi) and is not available or supported on ESXi hosts for performing upgrades.

vihostupdate is a deprecated legacy utility from older vSphere releases that has been superseded by esxcli and is no longer supported for current ESXi upgrade operations.

esxcfg refers to a family of host configuration commands (such as esxcfg-nics, esxcfg-vmknic) used for networking and storage configuration, not for performing software upgrades.