function renameCustomField($oldName, $newName)
    {
        $encodedOldName = urlencode(mb_convert_encoding($oldName, "UTF-8"));
        $encodedNewName = urlencode(mb_convert_encoding($newName, "UTF-8"));
        return $this->put("contacts/fields/custom/${encodedOldName}/${encodedNewName}");
    }