When using vSphere 5.5 if you upgrade your vCenter server to VM Hardware Version 10 then you will only be able to edit it via the vSphere Web Client. If the setting you are need to edit requires the VM to be powered down then you will not be able to use the vSphere Web Client as it requires the vCenter Service to be up and running.
Prior to VM Hardware version 10 once the vCenter VM is powered down you would connect your traditional C# vSphere Client directly to the ESX(i) host the vCenter VM is hosted on and then edit the settings. This is no longer possible with VM hardware version 10.
One option is to use PowerCLI, for example if you wanted to increase the memory on the vCenter VM and it is not configured for Hot-Add then you can use the following commands:
Set-VM <vCenter-VMName> -MemoryMB <New-Amount-of-Memory-in-MB>
For example if your powered down vCenter VM is called vCenter01 and is hosted on esxi01 and you want to increase the amount of memory to 12GB you would use the following commands
Set-VM vCenter01 –MemoryMB 12288
If you wanted to increase the CPUs to 4 you can use the following PowerCLI command after you have connected to the esxi host
Set-VM vCenter01 –NumCPU 4
While your vCenter server is down I would recommend configuring it for Hot-Add CPU and Memory using the following PowerCLI commands
$VM = Get-VM vCenter01
$hotadd = New-Object VMware.Vim.VirtualMachineConfigSpec
$hotadd.memoryHotAddEnabled = $true
$hotadd.cpuHotAddEnabled = $true
If you have VMware Workstation then another option is to use that to edit the settings. Within VMware Workstation 10 you have an option on the home screen to “Connect to a Remote Server”, use this to connect to the ESXi server your powered down vCenter VM is hosted on and then you will be presented with a list of virtual machines running on this host. You can then edit the settings of the powered down vCenter Server.