Check Current Kernel Version
$ sudo uname -r
List Old Kernels
$ sudo dpkg --list 'linux-image*'|awk '{ if ($1=="ii") print $2}'|grep -v `uname -r`
Uninstall Old Kernels
$ sudo apt-get purge Kernel_Image_Name
Remove no longer used dependencies
$ sudo apt-get autoremove
Update GRUB
$ sudo update-grub