ESXi 5.x uses the same installer for fresh installations and upgrades or migrations. If the installer finds an existing ESX/ESXi 4.x or ESXi 5.0 installation, it provides the option to upgrade or to do a fresh installation.

See vSphere Upgrade.

Depending on the disk layout of your system, the ESXi installer offers a choice between preserving or overwriting the VMFS datastore during installation.