how update mac os x 10.5.8 to mac os x 10.6.8

Go and Visit an Apple Store. There are no ways or purchasing outdated OS's from Apple online.

You have to go to the Apple Store, or an Apple retailer.

Of course, the other option would be to purchase online through Ebay, or perhaps craigslist.

You will now need to call Apple to get Snow Leopard. It is not available from the Store, but is available over the phone... for the same $29 ($49 family pack) as in the past. Way better than the $110-125 that it is selling for on Amazon now. 800 MY APPLE