How do I update my mac?

My mac is 10.5.6 and I just cannot update to 10.5.7 every time I try it spends 10 minutes or so downloading and then restarts or something and says something like an unknown error occurred contact your manufacturer or something like that and I just don't know what's wrong... and I am so pissed that after all the money I dropped on this thing to avoid these sucky problems I am reliving them after about 2 updates...