What is the correct way to convert observer to octane version of ember?

I was trying to convert all my ember-component into OCTANE version. But I got a bigger doubt. How can I convert the observer code into an OCTANE version? For example,

parent.hbs
 
 <Child @value= />

child.hbs

 <div></div>

child.js

  export default class ChildComponent extends Component {     
      /** Previous code: sample code only
        valueUpdate: observer('value', function() {
            this.newValue = this.value / 12 * 2;
        })
      */
  }


  How can I update the observer into octane way? Any idea please...

Note: I tried using '@observer' but it didn't work inside the component.