Try like this
$vmName='MyVM'
$left= (1..8|%{'{0:X2}'-f (Get-Random-Maximum255)}) -join' '
$right= (1..8|%{'{0:X2}'-f (Get-Random-Maximum255)}) -join' '
$newUuid=$left,$right-join'-'
$spec=New-ObjectVMware.Vim.VirtualMachineConfigSpec
$spec.uuid =$newUuid
$vm=Get-VM-Name$vmName
$vm.Extensiondata.ReconfigVM_Task($spec)