How to upgrade kernel on Ubuntu

You can find out which kernel version you are running by opening up a terminal and running the following command:

uname -r

To find out which version is the latest go to www.kernel.org.
To install the latest version you need to download the images. For that go to kernel.ubuntu.com/~kernel-ppa/mainline and open up the folder that matches the version number you found in www.kernel.org.
You need to download three files from the list. For 32 bit they are -

linux-headers-***-generic_***_i386.deb
linux-headers-***_***_all.deb
linux-image-***-generic_***_i386.deb

For 64 bit they are -

linux-headers-***-generic_***_amd64.deb
linux-headers-***_***_all.deb
linux-image-***-generic_***_amd64.deb

After downloading the files copy them to a temporary folder with no other deb files and run the following command from terminal -

sudo dpkg -i *.deb

This will install the new kernel. Restart you machine and uname -r to confirm you are running the latest kernel.
To uninstall the old kernel you could just use ubuntu-tweak. Install it with -

sudo apt-get install ubuntu-tweak

After installing ubuntu tweak, run it and open janitor. Do a clean and it will uninstall the old kernel for you.