Asp.net-mvc – Open xml replace text from word file and return memory stream using MVC

I have an word file that contain my specified pattern text {pattern} and I want to replace those pattern with new my string which was read from database. So I used open xml read stream from my docx template file the replace my pattern string then returned to stream which support to download file without create a temporary file. But when I opened it generated me error on docx file. Below is my example code

public ActionResult SearchAndReplace(string FilePath)
{
    MemoryStream mem = new MemoryStream(System.IO.File.ReadAllBytes(FilePath));
    using (WordprocessingDocument wordDoc = WordprocessingDocument.Open(mem, true))
    {
        string docText = null;
        using (StreamReader sr = new StreamReader(wordDoc.MainDocumentPart.GetStream()))
        {
            docText = sr.ReadToEnd();
        }

        Regex regexText = new Regex("Hello world!");
        docText = regexText.Replace(docText, "Hi Everyone!");

//Instead using this code below to write text back the original file. I write new string back to memory stream and return to a stream download file
        //using (StreamWriter sw = new //StreamWriter(wordDoc.MainDocumentPart.GetStream(FileMode.Create)))
        //{
        //    sw.Write(docText);
        //}

        using (StreamWriter sw = new StreamWriter(mem))
                    {
                        sw.Write(docText);
                    }
    }
    mem.Seek(0, SeekOrigin.Begin); 

    return File(mem, "application/octet-stream","download.docx"); //Return to download file
}

Please suggest me any solutions instead read a text from a word file and replace those expected pattern text then write data back to the original file. Are there any solutions replace text with WordprocessingDocument libary? How can I return to memory stream with validation docx file format?