Edit meta data does’t work with custom sql

I have a form that uploads posts to WordPress, and at the same time I can edit things like the meta data afterwards.
However using the WordPress edit_post_data works fine but a custom SQL query won’t work. I wonder why.

Works : update_post_meta($edit_id, 'lead_t', $_POST['lead_t']);

Doesn’t work :

$wpdb->query(
    "UPDATE wp_postmeta (post_id, meta_key, meta_value) 
    VALUES ('".$edit_id."', 'lead_t', '".$_POST['lead_t']."') 
    WHERE post_id = '".$edit_id."' AND meta_key = 'lead_t'"
);

Does anyone know how to troubleshoot this or debug to find the reason?

Try the following…

global $wpdb;

$wpdb->query(
    "
    UPDATE {$wpdb->prefix}postmeta 
    SET post_id = {$edit_id}, meta_key = '{$meta_key}', meta_value="{$meta_value}"
    WHERE post_id = {$edit_id} AND meta_key = '{$meta_key}'
    "
);

//or using prepare
$wpdb->query(
    $wpdb->prepare(
        "
        UPDATE {$wpdb->prefix}postmeta 
        SET post_id = %d, meta_key = %s, meta_value = %s
        WHERE post_id = %d AND meta_key = %s
        ",
        $edit_id,
        $meta_key, 
        $meta_value,
        $edit_id,
        $meta_key
    )
);

Also as a word of advice, set the following constants in your wp-config.php file to enable debugging/error logging.

define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);

Errors should be logged to a file wp-content/debug.log which should contain any errors relating to your query if it is malformed.