Either open up the Software Center and search for linux-generic-lts-saucy and xserver-xorg-lts-saucy or open a terminal and type the following:
sudo apt-get update
sudo apt-get install linux-generic-lts-saucy xserver-xorg-lts-saucy
Accept the installation and after the next reboot you will be using the latest kernel for 12.04 (as of 2/4/2014). For a newer kernel simply replace the name with the name of the latest distribution release. See here for a list:
http://en.wikipedia.org/wiki/List_of_Ubuntu_releases#Table_of_versions