Update record in SQL Table using LinqToSQL

using (DataContext dc = new DataContext())
            {
                eCreditCard ek = new eCreditCard();
                var query2 = (from y in dc.eUsers
                             where y.name + " " + y.lname == Session["LoggedUser"].ToString()
                             select y).SingleOrDefault();



                var query3 = (from x in dc.eCreditCards
                             where x.UserFK==query2.id
                             select x).SingleOrDefault();


if(query3==null) // so, if there are no credit card for this logged user, it save all required data!
{
                    ek.CreditCardNumber = txt_cnumber.Text;
                    ek.SecurityCode = txt_code.Text;
                    ek.ExDate = DateTime.Parse(txt_dateEx.Text);
                    ek.UserFK = query2.id;

                    OrdersController.SaveCard(ek);
}
else
{
/*here I have big problem! I want simple thing: if there are credit card for that user, just to update some records (security code, date of expiration..)
I have tried like this:*/
ek.CreditCardNumber = txt_cnumber.Text;
                    ek.SecurityCode = txt_code.Text;
                    ek.ExDate = DateTime.Parse(txt_dateEx.Text);
                    ek.UserFK = query2.id;
dc.SubmitChanges();
//but don`t work. Doesnt change the record.
}