Login to the admin end
Go to Catalog->Manage Products
Click on the edit action for the product you want to change the SKU.
Here you can find and modify the mandatory SKU field for your product.
Below code will help you to change the sku programmatically
$product = Mage::getModel('catalog/product')->load('product-id');
$product->getSku(); // Will give you the current sku valu of the product.
Hoping this will help you.