In a terminal, execute the following command:
dpkg -l 'linux-*' | sed '/^ii/!d;/'"$(uname -r | sed "s/\(.*\)-\([^0-9]\+\)/\1/")"'/d;s/^[^ ]* [^ ]* \([^ ]*\).*/\1/;/[0-9]/!d' | xargs sudo apt-get -y purge
This command lists all currently installed Linux kernels, then selects all of the ones that are currently not used and removes them.
Or:
apt-get autoremove
After, you should also sudo update-grub when you are finsihed.
sudo apt-get -f install
Done.