Datagridview: only Modified Cell + Current Row value will be saved to database

Question

I need help. First things first, I have datagridview which has columns(#, id, img, item_name, price, qty, discount, total), now all the columns are readonly except for qty column. What I want to happen is only the modified row will be saved to the database.

I tried

foreach (DataGridViewRow row in dataGridView1.Rows)
{ 
 int qty = Convert.ToInt32(row.Cells["qty"].Value.ToString()); 

 if (qty > 0) 
  { 
   con.Open();
   cmd = new SqlCommand("", con);         
   cmd.CommandType = CommandType.StoredProcedure;         
   cmd.Parameters.AddWithValue("@id",  Convert.ToInt32(dataGridView.Rows[e.RowIndex].Cells[1].Value.ToString()));
   and so on... 
  }
 con.Close(); 
}   

I am just a beginner in programming Thank you for your help in advance