Magento – How to Set default value for attributes

Basically, setting an attribute value to 'Use default values' means that you have to delete the row in the database for that attribute, for the specific product, for a store id.
Here is a simple solution that does that. It requires altering the database directly and some people will say that this is a big 'No-No' but it works.

$attrArray=array('thumbnail','small_image','image');
$products = array(170,171,172);
$stores = array(17,18,19);
$productsAsString = implode(',', $products);
$storesAsString = implode(',', $stores);
//get access to the resource
$resource = Mage::getSingleton('core/resource');
//get access to the db write connection
$connection = $resource->getConnection('core_write');
//model for retrieving attributes
$eavConfig = Mage::getModel('eav/config');
$tables = array();
//get the association between attribute ids and the tables where their values are stored
//group them by table name so you will run a single query for each table
foreach ($attrArray as $attributeCode){
    $attribute = $eavConfig->getAttribute('catalog_product', $attributeCode);
    if ($attribute){
        $tableName = $resource->getTableName('catalog/product') . '_' . $attribute->getBackendType();
        $tables[$tableName][] = $attribute->getId();
    }
}
//for each table delete the attribute values in the specified store for the specified products
foreach ($tables as $tableName => $attributeIds){
    $attributeIdsAsString = implode(',', $attributeIds);
    $q = "DELETE FROM {$tableName}
                WHERE
                    attribute_id IN ({$attributeIdsAsString}) AND
                    entity_id IN ({$productsAsString}) AND
                    store_id IN ({$storesAsString})";
    $connection->query($q);
}

This should be it. But in case I'm over-confident and this doesn't work, back-up your database first.

You can't update the default attributes in magento. If you want to change the default value of weight attribute then you can execute following query:

UPDATE `eav_attribute` SET `default_value` = '2' WHERE `eav_attribute`.`attribute_code`='weight';