How Can I Remove Syntax Error In Update Querry.

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C#

C#4.0

my Code is that in that code always occurred syntax error in update statement.in my combo box data is filled from the database.

code.

void shw_group()
       {


           try
           {
               s = "select * from create_group";

               da = new OleDbDataAdapter(s, cn);
               DataSet ds = new DataSet();
               da.Fill(ds, "set");
               cmb_group_name.DataSource = ds;
               cmb_group_name.DisplayMember = "set.group";
           }
           catch (Exception ea)
           {
               MessageBox.Show(ea.Message);
           }
       }

private void button1_Click(object sender, EventArgs e)
        {
            upd();
        }
        void upd()
        {
            string group_name;
            group_name = cmb_group_name.Text;
            string s = "update persn_info set group ="+group_name+" where sub_no="+ txt_sub_no.Text;
            cm = new OleDbCommand(s, cn);
            //cm.Parameters.AddWithValue("@group", group_name);


            cn.Open();
            cm.ExecuteNonQuery();
            cn.Close();
            MessageBox.Show("Subscriber information is updated", "Di-Count", MessageBoxButtons.OK, MessageBoxIcon.Information);
        }