How to change label text of the ActiveField?

Okay, just override attributeLabels in LoginForm.php:

/**
 * Returns the attribute labels.
 *
 * See Model class for more details
 *  
 * @return array attribute labels (name => label).
 */
public function attributeLabels()
{
    return [
        'username' => 'Логин',
        'password' => 'Пароль',
    ];
}

<?= $form->field($model, 'username')->textInput()->label('My superb label') ?>

http://www.yiiframework.com/doc-2.0/yii-bootstrap-activefield.html#label()-detail

there is an another cool way.

<?= $form->field($model, 'username')->textInput(['class'=>'field-class'])->label('Your Label',['class'=>'label-class']) ?>