i've found (thank you support vmware)
$vmName = 'computername'
$nicName = 'Network adapter 1'
$limit = 800
$vm = Get-VM -Name $vmName
$nic = Get-NetworkAdapter -VM $vm -Name $nicName
$spec = New-Object VMware.Vim.VirtualMachineConfigSpec
$dev = New-Object VMware.Vim.VirtualDeviceConfigSpec
$dev.Operation = [VMware.Vim.VirtualDeviceConfigSpecOperation]::edit
$dev.Device = $nic.ExtensionData
$dev.Device.ResourceAllocation.Limit = $limit
$spec.DeviceChange += $dev
$vm.ExtensionData.ReconfigVM($spec)
amicalement
Yannick Lecaille