glimmer

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  
  fields: Ember.A(),
 
  actions: {
 
    addField() {
      this.get('fields').insertAt(0, { label: '1' });
		},
    
		appendToField(index) {
      let field = this.get('fields').objectAt(index);
      Ember.set(field, 'label', `${field.label}1`);
		} 
	}
});

templates.application.hbs

{{#each fields as |field index|}}
	<p>{{field.label}} <button {{action 'appendToField' index}}>Append</button></p>
{{/each}}
<button {{action 'addField'}}>Add field</button>
<br>
<br>

twiddle.json

{
  "version": "0.10.4",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "alpha",
    "ember-data": "2.7.0",
    "ember-template-compiler": "alpha"
  },
  "addons": {}
}