Pre-built RPMs for kernels tend to lag behind the actual kernel itself. So you probably won't find any, but have a look on http://www.rpmfind.net/
by all means.
If you have gcc installed, then you can upgrade by downloading the tarball (.tar.bz2 file) from http://www.kernel.org.
Unpack it into /usr/src/linux (move whatever's there out of the way first!)
In a terminal, run:
cd /usr/src/linux; make cloneconfig xconfig
Take out only things you're sure you won't need, and make sure the processor type is correct. When you're sure you've got the configuration as you want it, save your changes and exit xconfig.
make modules modules_install install
Once that's finished, you just have to reboot your computer, and you will have a new kernel.
Tip: However you upgrade your kernel, always take a back-up of the old one first (files in /boot/ ending with the kernel version), and keep a rescue disk to hand. You can also set up your bootloader to load into the old kernel just in case.
You'll find more detailed instructions elsewhere. http://www.tldp.org
are good places to start.