write code for download file how can I click close button and reopen save dialog

Hi guys

   I am trying to write some code to download files which I did. when I clicked the file link button, a open/save/cancel dialog opens, when I clicked the open button, file opens in the browser. After I modified the file and click the close button, How do I open save dialog? I'ven trying to do these for days. Thanks

 protected void DownloadAssistant(string sFullFilePath)
        {
            try
            {
                FileInfo file = new FileInfo(sFullFilePath);
                if (file.Exists)
                {
     ...