Solution:
$modif_doc_bdd = $wpdb->query($wpdb->prepare("UPDATE `mod114_frmt_form_entry_meta_copie` SET `meta_value` = '$ValeurChampModif' WHERE `mod114_frmt_form_entry_meta_copie`.`meta_id` = $Champ_id;"));
if ( false === $modif_doc_bdd ) {
// There was an error.
} else {
// No error. You can check updated to see how many rows were changed.
}