using Rapidjson how to modify value of a particular key

 {
            "identifier":"Root",
            "type":"Structure",
            "uid":1,
            "address":"root",
            "parent":0
        },
//i tried to update address field using SetString and AddMember()
                 Value val;
                val.SetString(address.c_str(), address.length(),json_doc.GetAllocator());
                member.AddMember("address",val,json_doc.GetAllocator());

i tried to modify address's key value with new value which is stored in address variable