Hi PRA,

Refer the below sample.

C#

using System;
using System.Data;
using Word = Microsoft.Office.Interop.Word;

protected void Page_Load(object sender, EventArgs e)
{
    DataTable dt = new DataTable();
    dt.Columns.AddRange(new DataColumn[5] { 
        new DataColumn("LastName"), new DataColumn("FatherName"), new DataColumn("Adress"), new DataColumn("Name"), new DataColumn("Birthday") });
    dt.Rows.Add("Pulodov", "Abdulloevich", "city Dushanbe", "Rustam", "22.12.1987");
    if (dt.Rows.Count > 0)
    {
        object fileName = Server.MapPath("~/Files/Test.doc");
        Word.Application word = new Word.Application();
        Word.Document doc = new Word.Document();
        object missing = System.Type.Missing;
        try
        {
            doc = word.Documents.Open(ref fileName, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing,
                ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing);
            doc.Activate();
            foreach (Microsoft.Office.Interop.Word.Range docRange in doc.Words)
            {
                if (docRange.Text.Trim() == "#")
                {
                    docRange.Text = docRange.Text.Replace("#", "");
                }
                else if (docRange.Text.Trim() == "Ln")
                {
                    docRange.Text = docRange.Text.Replace("Ln", dt.Rows[0]["LastName"].ToString());
                }
                else if (docRange.Text.Trim() == "Fn")
                {
                    docRange.Text = docRange.Text.Replace("Fn", dt.Rows[0]["FatherName"].ToString());
                }
                else if (docRange.Text.Trim() == "Ad")
                {
                    docRange.Text = docRange.Text.Replace("Ad", dt.Rows[0]["Adress"].ToString());
                }
                else if (docRange.Text.Trim() == "Nm")
                {
                    docRange.Text = docRange.Text.Replace("Nm", dt.Rows[0]["Name"].ToString());
                }
                else if (docRange.Text.Trim() == "Bth")
                {
                    docRange.Text = docRange.Text.Replace("Bth", dt.Rows[0]["Birthday"].ToString());
                }
            }
            doc.SaveAs(@"C:\Users\Test\Desktop\" + dt.Rows[0]["Name"].ToString() + ".doc", missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing);
        }
        catch (Exception ex)
        {
            Response.Write(ex.Message);
        }
        finally
        {
            doc.Close(ref missing, ref missing, ref missing);
            ((Word._Application)word).Quit();
        }
    }
}

VB.Net

Imports Word = Microsoft.Office.Interop.Word
Imports System.Data

Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
    Dim dt As New DataTable()
    dt.Columns.AddRange(New DataColumn(4) {New DataColumn("LastName"), New DataColumn("FatherName"), New DataColumn("Adress"), New DataColumn("Name"), New DataColumn("Birthday")})
    dt.Rows.Add("Pulodov", "Abdulloevich", "city Dushanbe", "Rustam", "22.12.1987")
    If dt.Rows.Count > 0 Then
        Dim fileName As Object = Server.MapPath("~/Files/Test.doc")
        Dim word As New Word.Application()
        Dim doc As New Word.Document()
        Dim missing As Object = System.Type.Missing
        Try
            doc = word.Documents.Open(fileName, missing, missing, missing, missing, missing, _
                missing, missing, missing, missing, missing, missing, missing, missing, missing, missing)
            doc.Activate()
            For Each docRange As Microsoft.Office.Interop.Word.Range In doc.Words
                If docRange.Text.Trim() = "#" Then
                    docRange.Text = docRange.Text.Replace("#", "")
                ElseIf docRange.Text.Trim() = "Ln" Then
                    docRange.Text = docRange.Text.Replace("Ln", dt.Rows(0)("LastName").ToString())
                ElseIf docRange.Text.Trim() = "Fn" Then
                    docRange.Text = docRange.Text.Replace("Fn", dt.Rows(0)("FatherName").ToString())
                ElseIf docRange.Text.Trim() = "Ad" Then
                    docRange.Text = docRange.Text.Replace("Ad", dt.Rows(0)("Adress").ToString())
                ElseIf docRange.Text.Trim() = "Nm" Then
                    docRange.Text = docRange.Text.Replace("Nm", dt.Rows(0)("Name").ToString())
                ElseIf docRange.Text.Trim() = "Bth" Then
                    docRange.Text = docRange.Text.Replace("Bth", dt.Rows(0)("Birthday").ToString())
                End If
            Next
            doc.SaveAs("C:\Users\Test\Desktop\" & dt.Rows(0)("Name").ToString(), missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing)
        Catch ex As Exception
            Response.Write(ex.Message)
        Finally
            doc.Close(missing, missing, missing)
            DirectCast(word, Word._Application).Quit()
        End Try
    End If
End Sub

It will keep original file safe without writting any thing to it and Saves the document with a new name or format in the provided path.