How do I change static varbel

using System;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using Details;




namespace loginproject
{
    

    public partial class login : System.Web.UI.Page
    {
     
       
        protected void Page_Load(object sender, EventArgs e)
        {
            
            Session["username"] = Detail.username;
            Session["Password"] = Detail._password;
            if (!IsPostBack)
              
            {

                if (Request.Cookies["username"] != null)

                    txt_email.Text = Request.Cookies["username"].Value;

                if (Request.Cookies["Pass"] != null)

                    txt_pwd.Attributes.Add("value", Request.Cookies["Pass"].Value);
                if (Request.Cookies["username"] != null && Request.Cookies["Pass"] != null)
                    rember.Checked = true;
            }
          


        }

        protected void btn_submit_Click(object sender, EventArgs e)
        {
          


            if (txt_email.Text == Session["username"].ToString() && txt_pwd.Text == Session["Password"].ToString())
            {
                if (rember.Checked == true)
                {
                    Response.Cookies["username"].Value = txt_email.Text;
                    Response.Cookies["Pass"].Value = txt_pwd.Text;
                    Response.Cookies["username"].Expires = DateTime.Now.AddDays(15);
                    Response.Cookies["Pass"].Expires = DateTime.Now.AddDays(15);
                }
                else

                {

                    Response.Cookies["username"].Expires = DateTime.Now.AddDays(-1);

                    Response.Cookies["Pass"].Expires = DateTime.Now.AddDays(-1);

                }
                Response.Redirect("home.aspx");
            }
            else
            {
                ClientScript.RegisterStartupScript(Page.GetType(), "validation", "<script language='javascript'>alert('Invalid Username Or Password')</script>");
            }

        }
    }
}