Some code used for displaying the promo would be helpful.
Without that.
1) Create another field to hold the custom title, make it conditional on the select field having a value
2)
$custom_title = get_field('custom_title_field');
// maybe get the promo to show
// somewhere in that code
if (!empty($custom_title) {
echo $custom_title;
} else {
echo $whatever_should_be_shown_normally;
}