If you have decided to update all products' quantity and stock status at once, then you can write this code in any controller's action and hit that controller's action in browser.
$_products = Mage::getModel('catalog/product')->getCollection();
foreach($_products as $_product){
$product = Mage::getModel('catalog/product')->load($_product['entity_id']);
$stockItem = Mage::getModel('cataloginventory/stock_item')->loadByProduct($product);
$stockItem->setData('manage_stock', 1);
$stockItem->setData('is_in_stock', 1);
$stockItem->setData('use_config_notify_stock_qty', 0);
$stockItem->setData('qty', 10);
try{
$stockItem->save();
$product->save();
}catch(Exception $e){
Mage::log($e->getMessage(),null,'mohit.log');
}
}
I hope, my answer could solve your problem.
If not please comment. :)