You can create an ESX/ESXi host upgrade baseline by using the New Baseline wizard. You can create host baselines with already uploaded ESXi 5.5 images.
You can upload and manage ESXi images from the ESXi Images tab of the Update Manager Administration view.
Update Manager 5.5 supports upgrade from ESXi 4.x, ESXi 5.0 and ESXi 5.1 to ESXi 5.5 and migration from ESX 4.x to ESXi 5.5. You cannot use Update Manager to upgrade a host to ESXi 5.5 if the host was upgraded from ESX 3.x to ESX 4.x. Such hosts do not have sufficient free space in the /boot partition to support the Update Manager upgrade process. Use a scripted or interactive upgrade instead.
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.
If your vCenter Server system is part of a connected group in vCenter Linked Mode, and you have more than one Update Manager instance, host upgrade files that you upload and baselines that you create are not applicable to the hosts managed by other vCenter Server systems in the group. Upgrade files and baselines are specific for the Update Manager instance you select.