Although the command line option to upgrade is preferred in some cases, here is the option to upgrade your ESX/ESXi hosts from the vCenter .
First we have to import the correct upgrade file, which you downloaded from the VMware site. Depending on your current installation you can download different files.
To upgrade ESXi . . . → Read More: vSphere 4.1 is Here – Updating VMware vSphere ESX/ESXi Hypervisor to v4.1 through vCenter