Vue 数组和对象更新,但是页面没有刷新的解决方式

<ul>
   <li v-for="(item,index) in todos" :key="index">{{item.name}}</li>
 </ul>

data () {
  return {
   msg: 'Welcome to Your Vue.js App',
   todos: [{
    name: 'aa',
    age: 14
   }, {
    name: 'bb',
    age: 15
 
   }, {
    name: 'cc',
    age: 16
 
   }],
   obj: {name: 'lihui', age: 17}
  }
 },

methods: {
  changeTodos: function () {
   var _this = this
   _this.todos[0] = {
    name: 'zhangsan',
    age: 15
   }
   console.log(this.todos)
   /*
    this.$set(this.todos, 0, 'nn')
   this.$forceUpdate()
*/
  }

this.$set(this.todos,0,{name: 'zhangsan',age: 15})；或者对象this.$set(this.obj,'key',value);

this.$forceUpdate()