Replace

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  myArray: Ember.A(['foo', 'bar']),
  
  actions: {
    replace() {
      this.get('myArray').replace(0, 1, ['baz', 'wombat'])
    }
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
{{myArray.length}}
{{#each myArray as |item|}}
  <p>{{item}}</p>
{{/each}}
<br>
<br>
<button {{action "replace"}}>replace</button>

twiddle.json

{
  "version": "0.15.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.2.2",
    "ember-template-compiler": "3.2.2",
    "ember-testing": "3.2.2"
  },
  "addons": {
    "ember-data": "3.2.0"
  }
}