MacBook won't update

Issue.... I reloaded my mac OX 10.5. I go to software up-date, and it tells me there are no updates. I know this to be false. I can't update iTunes without the new safari, I can't up-date Safari without 10.5.8. How do I find what the latest OS I have?

wow... I looked at that page at least 10 times. How'd I miss that? Thanks.