if (0 < dataGridView1.SelectedRows.Count)
{
    int ix = dataGridView1.SelectedRows[0].Index + 1;
    if (ix < dataGridView1.Rows.Count)
    {
        foreach (DataGridViewRow r in dataGridView1.SelectedRows)
        {
            r.Selected = false;
        }
        dataGridView1.Rows[ix].Selected = true;
    }
}