how to move to next control in the form on pressing of down key in datagridview?

You will probably annoy your users by doing this but this would work

private void dataGridView1_KeyDown(object sender, KeyEventArgs e)
{
    if (e.KeyCode == Keys.Down)
    {
        textBox1.Focus(); // Replace this with the control you want
        e.Handled = true;
    }
}

Note the use of e.Handled = true; to stop the key press being processed by the DataGridView (and focus returning to it).

I would advise checking whereabouts you are in the DGV and only move to the next control if the user is on the last row e.g.

private void dataGridView1_KeyDown(object sender, KeyEventArgs e)
{
    if (e.KeyCode != Keys.Down || dataGridView1.CurrentCell.RowIndex != dataGridView1.Rows.Count - 1) return;

    textBox1.Focus();
    e.Handled = true;
}