数据库结构如下：

{"_id":"5b3","XM":"李四","SX":"2","ZH":[{"JC":"11","SX":"9","LD":{"HJ":"POP"}},{"JC":"22","SX":"2","LD":{"HJ":"opo"}},{"JC":"33","SX":"4","LD":{"HJ":"更新这里"}}]}

......

怎么更新ZH字段中 第三个对象 中LD字段中的HJ？