that worked for me after clearing selection, BeginEdit and change the girdview rows and end the Edit Mode.
if (dgvDetails.RowCount > 0)
{
dgvDetails.ClearSelection();
dgvDetails.BeginEdit(true);
foreach (DataGridViewRow dgvr in dgvDetails.Rows)
{
dgvr.Cells["cellName"].Value = true;
}
dgvDetails.EndEdit();
}