New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  init() {
		this.set('checkboxList', [{VALUE: "Content", CHECKBOX_ID: 1},{VALUE: "Size", CHECKBOX_ID: 2}]);
  },
  actions: {
    test() {
      this.set('checkboxList.0.VALUE', "Cont");
    }
  }
});

templates.application.hbs

<ul>
	{{#each checkboxList as |item|}}
  	<li>{{item.VALUE}} - {{item.CHECKBOX_ID}}</li>
  {{/each}}
</ul>

<button onclick={{action 'test'}}>test</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"
  }
}