The furthest you can go with 10.6.8 (as your profile indicates you have) is the 10.6.8 combo update as per the link below. Then follow that with software update to incorporate the security measures since introduced.
That is all you need to go on to Mountain Lion, if that is your ultimate aim. But ... make sure first that your machine is capable of going to a higher OS. It will accept the link I've given as you already have the basic Snow Leopard OS.