List old kernels (adapt output)
dpkg --list 'linux-image*'|awk '{ if ($1=="ii") print $2}'|grep -v
uname -r
...
linux-image-3.19.0-25-generic
linux-image-3.19.0-56-generic
...
Check existing
uname -a
Remove old kernels
apt-get purge linux-image-3.19.0-25-generic apt-get purge linux-image-3.19.0-56-generic
Remove eventual unused packages:
apt-get autoremove
Update grub kernel list
update-grub