param
(
    [string]$AppCatalogUrl,
    [string]$TargetSiteUrl,
    [string]$AppName
)

if ((Get-PSSnapin "Microsoft.SharePoint.PowerShell" -ErrorAction SilentlyContinue) -eq $null) {
    Add-PSSnapin "Microsoft.SharePoint.PowerShell";
}

$appCatalog = Get-SPWeb $AppCatalogUrl;
if ($appCatalog -eq $null) {
    throw "App Catalog Site does not exist at ${AppCatalogUrl}";
}

$targetSite = Get-SPWeb $TargetSiteUrl;
if ($targetSite -eq $null) {
    throw "Target Site does not exist at ${TargetSiteUrl}";
}

$appList = $appCatalog.Lists.TryGetList("Apps for SharePoint");
$appCatalogAppItem = $null;
if ($appList -ne $null) {
    $appCatalogAppItem = $appList.Items | ?{$_.Title -eq $AppName};
    if ($appCatalogAppItem -eq $null){
        throw "{$AppName} does not exist in the app catalog";
    }
}

$appGuid = $appCatalogAppItem["Product ID"].Replace("{","").Replace("}","");
$siteCollection = $targetSite.Site;
foreach ($web in $siteCollection.AllWebs)
{
    $appInstances = Get-SPAppInstance -Web $web;
    foreach ($instance in $appInstances)
    {
        if ($instance.App.ProductId -eq $appGuid)
        {
            $stream = $appCatalogAppItem.File.OpenBinaryStream();
            $instance.Upgrade($stream);
            $stream.Close();
        }
    }
}