In magento sometimes we are required to change the attribute values of the products. 
To do so we can work upon the event catalog_product_load_after.

Lets see how we can do it:

1. Create a module with namespace as Custom and module as ProductInfo.

2. Then Create a file config.xml in etc folder in our module.

3. Afterwards create a file Observer.php in Model folder.

Now, in our config.xml file at the path Custom/ProductInfo/etc/config.xml lest change the product name for the same write the below code in it :

<?xml version="1.0"?>
<config>

 <modules>
  <Custom_ProductInfo>
   <version>0.1.0</version>
  </Custom_ProductInfo>
 </modules>

 <global>

 <events>
  <catalog_product_load_after>
   <observers>
    <Custom_ProductInfo_Model_Observer>
     <class>custom_productInfo/observer</class>
     <method>ProductInfo</method>
    </Custom_ProductInfo_Model_Observer>
   </observers>
  </Custom_ProductInfo_load_after>
 </events>
</global>

</config>

In the above code we have defined our module with the version 0.1.0 and 
then in global we have called our Observer at the event catalog_product_load_after 

Now in our Observer at the path Custom/ProductInfo/Model/Observer.php write the below code:

<?php

class Custom_ProductInfo_Model_Observer {

public function ProductInfo($observer) {

    $product = $observer->getProduct();
    $product = $observer->getData('product');

    $product->setName($product->getName().' New Name !');
  }

}

In the above code we are loading the product first and then with the setName() magento method we have set up the new name as " New Name ! ". That's it we are done.
In the same way we can change any attribute value of products.