Removing old kernels

check the kernel you’re using:

$ uname -r


Do NOT delete that kernel!


List the old kernels:

$ sudo dpkg --list 'linux-image*'|awk '{ if ($1=="ii") print $2}'|grep -v `uname -r`



Remove the old kernels one by one:

$ sudo apt-get purge linux-image-3.19.0-25-generic


Remove the packages that are not needed anymore:

$ sudo apt-get autoremove


Update the grub kernel list:

$ sudo update-grub


References :
Safest way to clean up boot partition – Ubuntu 14.04LTS-x64