help to write a custom php script to settingup yoast priority to pages or posts

The solution should is this:

add_filter( 'wpseo_xml_sitemap_post_priority', 'my_custom_post_xml_priority', 10, 3 );

function my_custom_post_xml_priority( $return, $type, $post) {

    if($type == 'page') {
        switch ($post->ID) {
        case '8':
        case '395':
        case '342':
            $return = 0.9;
            break;
        case '5':
            $return = 1.00;
            break;
        case '620':
        case '703':
        case '603':
        case '688':
        case '695':
        case '614':
        case '684':
        case '639':
        case '628':
        case '539':
        case '542':
        case '521':
        case '509':
        case '517':
        case '528':
        case '533':
        case '536':
        case '548':
        case '545':
        case '525':
        case '551':
            $return = 0.7;
            break;
        default:
            $return = 0.5;
            break;
        }
    } elseif ($type == 'post') {
        $return = 0.8;
    }

    return $return;
}

I think that $post is an WP_Post object, then you will have to compare the property ID on the switch.