<?= $form->field($model, 'username')->textInput()->label('My superb label') ?>
http://www.yiiframework.com/doc-2.0/yii-bootstrap-activefield.html#label()-detail
<?= $form->field($model, 'username')->textInput()->label('My superb label') ?>
http://www.yiiframework.com/doc-2.0/yii-bootstrap-activefield.html#label()-detail