add_action('acf/save_post', 'acf_field_to_title', 20);
function acf_field_to_title($post_id) {
remove_filter('acf/save_post', 'acf_field_to_title', 20);
$title = get_field('your-field', $post_id);
if ($title) {
$post = get_post($post_id);
$post->post-title = $title;
wp_update_post($post);
}
add_action('acf/save_post', 'acf_field_to_title', 20);
}