how updating OSx 10.6.8 to higher version in macbook white 2008?

I have a macbook withe with these specifications: https://support.apple.com/kb/SP5?viewlocale=pt_BR&locale=pt_BR

However, it has 4GB of memory because upgrade.

I would like to know how I can update the OSx! He's with the snow leopard, but not can not update the apple store. Alert says not there available updating ...