You can upgrade or migrate the hosts in your environment to ESXi 5.5 by using host upgrade baselines. To create a host upgrade baseline, you must first upload at least one ESXi 5.5 .iso image to the Update Manager repository.
With Update Manager 5.5 you can upgrade or migrate hosts that are running ESX/ESXi 4.x, ESXi 5.0 or ESXi 5.1 to ESXi 5.5. Host upgrades to ESX/ESXi 4.x, ESXi 5.0 or ESXi 5.1 are not supported.
Before uploading ESXi images, obtain the image files from the VMware Web site or another source. You can create custom ESXi images that contain third-party VIBs by using ESXi Image Builder CLI. For more information, see Using vSphere ESXi Image Builder CLI.
You can upload and manage ESXi images from the ESXi Images tab of the Update Manager Administration view.
ESXi images that you import are kept in the Update Manager repository. You can include ESXi images in host upgrade baselines. To delete an ESXi image from the Update Manager repository, first you must delete the upgrade baseline that contains it. After you delete the baseline, you can delete the image from the ESXi Images tab.
For more information about importing ESXi images and creating host upgrade baselines, see Create a Host Upgrade Baseline.