Am using update manager (VUM) 5.5.0.22432 version.
My esxi 5.1 host was installed with fresh esxi 5.1 ISO, in my organization till now whenever there is need for esxi upgrade , we use will install it with custom iso instead of upgrading from update manager.
These is the first time we are trying to upgrade from VUM. So the existing one host 5.1 is fresh install.