In this example I want to show how to update the modify by and
modified date using PowerShell.
Sometimes it may happen that exiting item or items we need
to update with the user name instead of our name, for that we can achieve using
power shell easily.
Here I am updating the Last modified name and the date with
name for a particular items based on the item ID.
$web = Get-SPWeb http://dotnetsharepoint.com/sites/dotnet/sharepoint2013
$list = $web.lists["UpdateModified"]
#23 is my item id,i am update for a particular item only
$item = $list.items.getitembyid(23)
$username = $Web.EnsureUser(domain/username)
$item["Editor"] = $username
$item["Modified"] = "7/17/2015 8:00 AM"
$item.Update()
$list.Update()
$web.Dispose()