元の配列オブジェクトの特定の位置のデータを変更します.

//  a
[{
	"num": "20181101",
	"name": "  ",
	"age": 12
}, {
	"num": "20181102",
	"name": "  ",
	"age": 22
}]

const newData = a;
let num = "20181102";
const target = newData.filter((item,index) => num === item.num)[0];
if (target) {
   target['sex'] = " ";
   console.log("    newData:",newData);
}