Alter Link field in node

  if ($node ->bundle() == 'page' && 
      $variables['elements']['#view_mode'] == 'teaser') {
 
    $opts = [
      'attributes' => [
        'class' => [
          'btn',
          'btn-primary',
          'btn-sm'
        ]
      ]
    ];
 
    if ($variables['content']['field_link'][0]) {
      $variables['content']['field_link'][0]['#url']->setOptions($opts);
    }
  }