it's very easy to update your kernel...
only download the patch and move it to /usr/src
go to your kernel folder (let's say it's at /usr/src/linux) and do this :
bunzip2 ../patch-2.6.21.bz2 && make && make install && make modules_install
it will ask you questions about the new features and you will answer y, n or m (module). if you don't know what to answer, press enter.
restart + good luck.