Change attributes magento

I use custom attributes for my products and I am working on my script, which get attributes and after change them value. I have done selecting code but I dont know how can I do code, which change value.

    $mageFilename = 'app/Mage.php';
    require_once $mageFilename;
    $collection = Mage::getModel('catalog/product')->getCollection()
    ->addAttributeToFilter('manufacturer', '3')

    foreach ($collection as $product) {

      echo $product->getName();
      echo $product->getResource()->getAttribute('cena_balenia_czk')->getFrontend()->getValue($product);
      echo '-';
      echo $product->getResource()->getAttribute('czk')->getFrontend()->getValue($product);
      echo '<br>';