Update a list by its index

I'm working with Windows Forms I have a DataGridView object which people with a list. What I need is that when you select an element of the grid take the index and with that index do the search in the list to update a property that you have.

private void dText_keyDown(object sender, KeyEventArgs e)
    {
        int rowIndex = ((DataGridViewTextBoxEditingControl) (sender)).EditingControlRowIndex;
        _rowIndex = rowIndex;
        if (e.Alt && e.KeyCode == Keys.D)
        {
            var frm = CompositionRoot.Resolve<FrmCantidad>();
            _hub.Subscribe<SelectedCantidad>(OnCantidadSelected);
            frm.ShowDialog();
            _hub.ClearSubscriptions();
        }
    }

    private void OnCantidadSelected(SelectedCantidad obj)
    {
        if (!string.IsNullOrEmpty(obj.Valor))
        {
            var list = _saVenta.MostrarVentas().ToList();
            if (list)
            {

            }
                .
        }
    }

The DGV charges it by assigning its DataSource property to the list.