mut demo

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  foo: '5',
  bar: '100',
  baz: '0'
});

templates.application.hbs

Foo {{foo}}<br>
Bar {{bar}}<br>
<p>
<button {{action (mut baz) foo}}>Set to foo</button>
<button {{action (mut baz) bar}}>Set to bar</button>
</p>
<p>
Baz: {{baz}}
</p>
<br>
<br>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}