The UpdateVirtualSwitch method came up in a previous discussion. See  Re: Nic Teaming Policy.

In the end I discovered that the way the method is described in the SDK API Reference Guide doesn't correspond with the reality.

The method needs most (if not all) of the properties in the HostVirtualSwitchSpec to be set.

You can go the long way. Something like this

param ( [string]$VMHostName, [string]$VSwitchName )
 
$vmhost = Get-VMHost $VMHostName
$hostview = $vmhost | Get-View
$ns = Get-View -Id $hostview.ConfigManager.NetworkSystem
$vsSpec = New-Object VMware.Vim.HostVirtualSwitchSpec
$vsSPec.Bridge = New-Object VMware.Vim.HostVirtualSwitchBondBridge
$vsSPec.Bridge.Beacon = New-Object VMware.Vim.HostVirtualSwitchBeaconConfig
$vsSPec.Bridge.Beacon.Interval = 1
$vsSPec.Bridge.NicDevice = ("vmnic2","vmnic1")
$vsSpec.Mtu = 0
$vsSpec.numPorts = 64
$vsSpec.Policy = New-Object VMware.Vim.HostNetworkPolicy
$vsSpec.Policy.NicTeaming = New-Object VMware.Vim.HostNicTeamingPolicy
$vsSpec.Policy.NicTeaming.FailureCriteria = New-Object VMware.Vim.HostNicFailureCriteria
$vsSpec.Policy.NicTeaming.FailureCriteria.checkBeacon = $false
$vsSpec.Policy.NicTeaming.FailureCriteria.checkDuplex = $false
$vsSpec.Policy.NicTeaming.FailureCriteria.checkErrorPercent = $false
$vsSpec.Policy.NicTeaming.FailureCriteria.checkSpeed = "minimum"
$vsSpec.Policy.NicTeaming.FailureCriteria.fullDuplex = $false
$vsSpec.Policy.NicTeaming.FailureCriteria.Percentage = 0
$vsSpec.Policy.NicTeaming.FailureCriteria.Speed = 10
$vsSpec.Policy.NicTeaming.NicOrder = New-Object VMware.Vim.HostNicOrderPolicy
$vsSpec.Policy.NicTeaming.NicOrder.ActiveNic = ("vmnic1","vmnic2")
$vsSpec.Policy.NicTeaming.NotifySwitches = $true
$vsSpec.Policy.NicTeaming.Policy = "loadbalance_srcmac"
$vsSpec.Policy.NicTeaming.ReversePolicy = $true
$vsSpec.Policy.NicTeaming.RollingOrder = $false
$vsSpec.Policy.OffloadPolicy = New-Object VMware.Vim.HostNetOffloadCapabilities
$vsSpec.Policy.OffloadPolicy.CsumOffload = $true
$vsSpec.Policy.OffloadPolicy.TcpSegmentation = $true
$vsSpec.Policy.OffloadPolicy.ZeroCopyXmit = $true
$vsSpec.Policy.Security = New-Object VMware.Vim.HostNetworkSecurityPolicy
$vsSpec.Policy.Security.AllowPromiscuous = $true
$vsSpec.Policy.Security.ForgedTransmits = $true
$vsSpec.Policy.Security.MacChanges = $true
$vsSpec.Policy.ShapingPolicy = New-Object VMware.Vim.HostNetworkTrafficShapingPolicy
$vsSpec.Policy.ShapingPolicy.AverageBandwidth = 0
$vsSpec.Policy.ShapingPolicy.BurstSize = 0
$vsSpec.Policy.ShapingPolicy.Enabled = $false
$vsSpec.Policy.ShapingPolicy.PeakBandwidth = 0

$ns.UpdateVirtualSwitch($VSwitchName,$vsSpec)

You would need some extra params to do it like this (i.e. for the NICs).

Or you could go the short(er) way.

param ( [string]$VMHostName, [string]$VSwitchName )

$esxImpl = Get-VMHost -Name $VMHostName
$esx = $esxImpl | Get-View

foreach($sw in $esx.Config.Network.Vswitch){
  if($sw.Name -eq $VSwitchName){break}
}

$vsSpec = $sw.Spec
$vsSpec.Policy.NicTeaming.Policy = "loadbalance_srcmac"
$vsSpec.Policy.Security.AllowPromiscuous = $true

$ns.UpdateVirtualSwitch($VSwitchName,$vsSpec)