How to remove old kernels from Ubuntu 12.04?

You might have upgraded from a previous Ubuntu release or may be your kernal is new and old ones still residing in your system. Some times we need to free space. Do the following to remove old kernels:

Open terminal and check your current kernel in use:
$ uname -r
NOTE: You must not remove this one.

To list all installed kernels on your system:
$ dpkg –list | grep linux-image

Identify all kernels that are lower than your current kernel and you want to remove. Now issue the following command to remove:
$ sudo apt-get purge linux-image-x.x.x-xx-generic-pae
NOTE: Replace all 'x' acording to your kernel version which you want to remove.

Finally, run grub2:
$ sudo update-grub2

Now reboot your system and you are all done.