How to Update the record of user

protected void btn_profileEdit_Click(object sender, EventArgs e)
        {
            tbox_profileDOB.Enabled = true;
            tbox_profileEmail.Enabled = true;
            tbox_profileFName.Enabled = true;
            tbox_profileGender.Enabled = true;
            tbox_profileGroupId.Enabled = true;
            tbox_profileLName.Enabled = true;
            tbox_profileUserName.Enabled = true;

        }

        protected void btn_profileSaveChanges_Click(object sender, EventArgs e)
        {  
            if (Session["userName"] != null)
            {
                string userName = Session["userName"].ToString();
                var userDetails = (from user in db.web_users
                                   join stats in db.user_stats on user.Id equals stats.UserId
                                   where user.UserName == userName
                                   select new { user, stats }).FirstOrDefault();

                //db.web_users.Remove(userDetails.user);
                //db.user_stats.Remove(userDetails.stats);
                
                userDetails.user.DOB = Convert.ToDateTime(tbox_profileDOB.Text);
                userDetails.user.Email = tbox_profileEmail.Text;
                userDetails.user.FirstName = tbox_profileFName.Text;
                userDetails.user.Gender = tbox_profileGender.Text;
                userDetails.user.LastName = tbox_profileLName.Text;
                userDetails.user.UserName = tbox_profileUserName.Text;
                userDetails.stats.GroupId = Convert.ToInt32(tbox_profileGroupId.Text);

                //db.web_users.Add(userDetails.user);
                //db.user_stats.Add(userDetails.stats);
                db.SaveChanges();
                
                tbox_profileDOB.Enabled = false;
                tbox_profileEmail.Enabled = false;
                tbox_profileFName.Enabled = false;
                tbox_profileGender.Enabled = false;
                tbox_profileGroupId.Enabled = false;
                tbox_profileLName.Enabled = false;
                tbox_profileUserName.Enabled = false;
             }
             else
             {
                 Response.Redirect("Login.aspx");
             }
        }