how can i set checkbox in header template in gridview? inc# side

I want to change the checkbox value in grid view in cs code not in asp.net side? How can i set the it?

I tried it

    while (reader.Read())
    {
        foreach (TableCell headerCell in GridView1.HeaderRow.Cells)
        {
            // or access Controls with index
            //     headerCell.Controls[INDEX]
            headerCell.FindControl("c") as CheckBox = true;

        }
    }