New Twiddle

components.my-user.js

import Ember from 'ember';

export default Ember.Component.extend({
  
});

controllers.application.js

import Ember from 'ember';
import ArrayProxy from '@ember/array/proxy';
import { A } from '@ember/array';
import { set } from '@ember/object';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  users: [
    {
    	first: 'foo',
    	last: 'bar'
  	},
  ],
  
  actions: {
  	updateMe(data) {
      let old = this.get('users');
      console.log(old[0]);
      console.log(old.get('firstObject'));
      console.log(this.get('users.firstObject'));
//      old.objectAt(0).set('first', data);
      
      //set(old[0], 'first', data);
//      set(old[0], 'first', data);
      old.replace(0, 1, [{first: data, last: 'xxx'}]);
//  		this.set('users.firstObject.first', data);
		}
    
  }                                   
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{my-user users=users updateUser=(action "updateMe")}}
<br>
<br>

templates.components.my-user.hbs

{{#each users as |user|}}
	{{user.first}}
	{{user.last}}
{{/each}}

<hr/>
<button {{action updateUser "bar"}}>click me</button>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}