Add-PsSnapin Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue
# define Variables
$SiteUrl = “http://portal.contoso.com/subsite”
$Library = “Documents”
$fileName = “FileName.docx”
$web = Get-SPWeb $SiteUrl
$Library = $web.GetFolder($Library)
$File = $Library.Files | ? {$_.name -eq $fileName}

$File.CheckIn(“Undo Checkout by force”)

$File.UndoCheckOut()
$File.Update()