To update the kernel you can issue the command
This will actually update everything. I try to do this weekly to stay updated. If you haven't done it in a while it will take a while to download everything. I'm not sure but I think 'yum -y update kernel' will work (or kernel-smp) if you only want to update the kernel. I know there have been some fixes to flaws in the system so you may want to update everything.