To convert our database into excel file as a backup on the button Click Event.

Add this header name in your cs file:
using System;
using System.Collections;
using System.ComponentModel;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Drawing;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;

In your export to excel button click event write this code:

SqlConnection con;

con = new SqlConnection(ConfigurationManager.ConnectionStrings["con"].ConnectionString);
        con.Open();
        SqlCommand cmd = new SqlCommand("select * from tablename", con);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataTable ds = new DataTable(); try
        {
            da.Fill(ds);
        }
        catch (Exception ex)
        {
            System.Diagnostics.Trace.WriteLine(ex.Message);
            throw ex;
        }
        finally
        {
            con.Close();
        }
        Convert(ds, Response);
    }
public static void Convert(DataTable ds, HttpResponse Res)
    {
        Res.Clear();
        Res.Charset = "";
        Res.ContentType = "application/vnd.ms-excel";
        Res.AddHeader("Content-Disposition", "attachment;filename=\\tablename.xls");
        System.IO.StringWriter stringWrite = new System.IO.StringWriter();
        System.Web.UI.HtmlTextWriter htmlWrite = new System.Web.UI.HtmlTextWriter(stringWrite);
        System.Web.UI.WebControls.GridView dg = new System.Web.UI.WebControls.GridView();
        dg.DataSource = ds;
        dg.DataBind();
        dg.RenderControl(htmlWrite);
        Res.Write(stringWrite.ToString());
        Res.End();
    }

This Convert method convert our database to Excel format .

When you run this code and click on Export button it download one .xls file in which our table data will be saved.

Happy Coding.....

Thank You.....