update query in linq in asp.net

MyLinqDataClassesDataContext mylinq = new MyLinqDataClassesDataContext();

var user =( from u in mylinq.userCredentials

                   where u.uname_db.Contains(TextBox1.Text)

                    select u).SingleOrDefault();

        user.password_db = TextBox2.Text;

        mylinq.SubmitChanges();