So ESXi 4.0 is now available and you want to get cracking learning everything you can about it. You go to upgrade it in your home environment or your test lab and you realize you can’t do anything because you don’t have vCenter. What do you do?
Never fear! You can upgrade it anyway! What’s the secret? The vSphere host update utility! You can gain access to it by downloading the vSphere upgrade zip file from the VMware website first and then extracting it using the program 7zip. 7zip will allow you to drill down into the following path: VMware-viclient.vib\data.tar.gz\data.tar\.\4.0.0\client\VMware-viclient.exe to extract the VMware client install file to install the vSphere client.
Once you install the client it will ask you to install the host update utility and then after that is done you can just launch the utility. Point it at your server and tell it to upgrade after putting your ESXi host in maintenance mode and presto, your ESXi server has been upgrade to 4.0. As per usual I always recommend you do backups before you attempt any upgrading or patching.
There is detailed instructions with screenshots you can follow here.