آموزش ویرایش اطلاعات با Entity framework

string studid=dataGridView1.SelectedCells[0].Value.ToString();   testEntities db = new testEntities();
student st = db.student.Where(p => p.stid == studid).First();
textBox12.Text = st.stid;
  textBox13.Text = st.name;
   textBox11.Text=st.famil;
   textBox6.Text = st.avg.ToString();

string studid = textBox12.Text.Trim();
  testEntities db = new testEntities();
  student st = db.student.Where(p => p.stid == studid).First();
    st.name=textBox13.Text;
  st.famil=textBox11.Text;
  st.avg =Convert.ToDouble(textBox6.Text);  
db.SaveChanges();  
dataGridView1.DataSource = db.student.Select(p => p);