Angular FormArray Insert

I am trying to update a specific value on an formArray using insert(). I initialize form on ngOninit() like this:

this.myForm = this.fb.group({
    exercises: this.fb.array([]),
    type: new FormControl(),
    day: new FormControl(),
    sets: this.fb.array([]),
    reps: this.fb.array([]),
});

I have an input where on change i call the below function but when i try to insert a new value with the array's index I get the value pushed.

onSetsChange(sets, index){
  var setsFormArray = <FormArray>this.myForm.controls.sets;
  setsFormArray.insert(index, new FormControl(sets));
}

The html code is below:

<div class="col-md-3">
  <select class="form-control input-sm" (change)="onSetsChange($event.target.value, exerciseIndex)">
     <option *ngFor="let set of sets; let i = index;">{{set}}</option>
  </select>
</div>

The exerciseIndex I pass is from a loop that it doesn't show up.

What i am doing wrong and the value it's not updating? Thank you

Try using patchValue :-

public ngOnInit() {
    const arr = new FormArray([
                new FormControl(),
                new FormControl()
            ]);

    let control = arr.controls[0];
    setTimeout(() => control.patchValue("new-val"), 250); // Set value to "new-val"
    setTimeout(() => console.log(control.value), 350); // Logs "new-val"
}