title: Patch value of a form array at a specific index
body: <p>I have constructed my form like this.</p>
  
  <pre><code>this.myForm = this.fBuilder.group({
    openingHoursForm: this.fBuilder.array([])
  });
  
  this.openingHoursArray = data.body.OpeningHours;
  
  this.openingHoursArray.forEach(element =&gt; {
  
  (&lt;FormArray&gt;this.myForm.get('openingHoursForm')).push(this.fBuilder.group({
                              Id: [element.Id],
                              Heading: [element.Heading],
                              Subheading: [element.Subheading],
                              When: [element.When],
                              Times: [element.Times],
                              Emphasis: [element.Emphasis],
                              SortOrder: [element.SortOrder]
                      }));
                  });
  </code></pre>
  
  <p>But how do I patch the value of "SortOrder" at a specific index. I have this but it's not working:</p>
  
  <pre><code>moveRowUp(currentIndex,fixture)
      {
          const control = &lt;FormArray&gt;this.myForm.controls['openingHoursForm'];
          control.at(currentIndex)['SortOrder'].patchValue(currentIndex + 1);
          console.log(this.myForm);
      }
  </code></pre>