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When using an AJAX handler in your Controller, you can update a single form field as shown below:

```
public function onYourAjaxHandler($id = null, $context = null)
{
   if (is_null($id)) {
		   $model = $this->formCreateModelObject();
		   $model = $this->formExtendModel($model) ?: $model;
   } else {
		   $model = $this->formFindModelObject($id);
   }
	 
   $fieldName = 'fieldToUpdate';
   $fieldId = sprintf("#Form-field-%s-%s-group", class_basename(get_class($model)), $fieldName);

$model->{$fielName} = "new value";
   $this->initForm($model, $context);

$fieldMarkup = $this->formRenderField($fieldName, ['useContainer'=>false]);

return [
      $fieldId => $fieldMarkup,
   ];
}

public function create_onYourAjaxHandler()
{
   return $this->onYourAjaxHandler(context:"create");
}

public function update_onYourAjaxHandler($id)
{
   return $this->onYourAjaxHandler($id, context:"update");
}
```

You can find the Id of the Form Field Group by inspecting your Form's HTML code using the Debug Tool Inspector in your browser, but the above code should be able to generate the Id based on the value you assign to `$fieldName`.

If you need to *replace* the field container, set `useContainer => false`. If you need to *keep* the container, set `useContainer => true`.

Tip: if you want to update a repeater field from another custom formwidget on the same form, just use this instead:
```
$controller = $this->controller;
$controller->initForm($model);
$fieldMarkup = $controller->formRenderField('fieldToUpdate', ['useContainer'=>false]);
```
And return the result shown previously.

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