How to uncheck top level checkbox in datagrid

CheckBox chkbox=new CheckBox( );
        DataTable dt = new DataTable( );

private void Form1_Load(object sender, EventArgs e)
        {
            //CheckBox chkbox=new CheckBox( );
            dataGridView1.AllowUserToAddRows = false;
            
            dataGridView1.Columns.Clear( );
            DataGridViewCheckBoxColumn colCB = new DataGridViewCheckBoxColumn( );            
            colCB.Name = "chkcol";
            colCB.HeaderText = "";
           
            colCB.HeaderCell.Style.Alignment = DataGridViewContentAlignment.MiddleCenter;
            dataGridView1.Columns.Add( colCB );

            Rectangle rect = this.dataGridView1.GetCellDisplayRectangle( 0, -1, true );
           
            chkbox.Size = new Size( 18, 18 );
            rect.Offset( 43, 2 );
            chkbox.Location = rect.Location;
            chkbox.CheckedChanged += chkBoxChange;
            this.dataGridView1.Controls.Add( chkbox );

            DataRow dr = default( DataRow );

            //Declare Column names
            dt.Columns.Add( "eno" );
            dt.Columns.Add( "empname" );
            dt.Columns.Add( "sal" );

            //Create rows with data
            dr = dt.NewRow( );
            dr["eno"] = 101;
            dr["empname"] = "test1";
            dr["sal"] = 9000;
            dt.Rows.Add( dr );

            dr = dt.NewRow( );
            dr["eno"] = 102;
            dr["empname"] = "test2";
            dr["sal"] = 15000;
            dt.Rows.Add( dr );

            dr = dt.NewRow( );
            dr["eno"] = 103;
            dr["empname"] = "test3";
            dr["sal"] = 20000;
            dt.Rows.Add( dr );

            //Bind that datatable data into the datagridview
            dataGridView1.DataSource = dt;
        }

private void dataGridView1_CellContentClick(object sender, DataGridViewCellEventArgs e)
       {
           double k = 0.0;
           if ( e.ColumnIndex == 0 )
           {
               dataGridView1.CommitEdit( DataGridViewDataErrorContexts.Commit );
               for ( int i = 0 ; i <= dataGridView1.RowCount - 1 ; i++ )
               {
                   //chkcol is checkbox column
                   if ( Convert.ToBoolean( dataGridView1.Rows[i].Cells["chkcol"].Value ) == true )
                   {
                       k = k + Convert.ToDouble( dataGridView1.Rows[i].Cells[3].Value );      //3 is salary column
                   }
               }
               //This one is total display textbox
               //label1.Text = k.ToString( );
           }
       }