How to upgrade from Mac OS x 10.5.8

How do I upgrade from Mac OS x 10.5.8 to OS x Mountain Lion??

Model Name: MacBook Pro

Model Identifier: MacBookPro5,5

Processor Name: Intel Core 2 Duo

Mac OS X 10.5.8

I got the OS X 10.6 Snow Leopard but it won't let me install it because it asks from the operating system to be Max OS X version 10.6

Any help is appreciated!