Mac OS X 10.6 is not available yet. !0.5 is the current version. If your Mac meets the minimum requirements (if it is still running 10.2 it may not) you will need to buy a copy of Mac OS X Leopard (10.5) and install it.
Mac OS X 10. 4 (Tiger) can be updated via Software Update found in the Apple menu. The highest version available is 10.4.11. To upgrade further will require purchasing a copy of either Mac OS X 10.5...