How to remove kernels from previous release?

Open terminal and check your current kernel:

uname -r
DO NOT REMOVE THIS KERNEL!

Next, type the command below to view / list all installed kernels on your system.

dpkg --list | grep linux-image
Find all the kernels that lower than your current kernel. When you know which kernel to remove, continue below to remove it. Run the commands below to remove the kernel you selected.

sudo apt-get purge linux-image-x.x.x.x-generic
or
sudo apt-get purge linux-image-3.2.0-3{0..5}-generic

Finally, run the commands below to update grub2

sudo update-grub2
Reboot your system.