Seems duplicated with : What is the safest way to clean up /boot partition?
Get the list of kernel images (this command will show installed kernels except the currently running one) :
sudo dpkg --list 'linux-image*'|awk '{ if ($1=="ii") print $2}'|grep -v `uname -r`
Note the two newest versions in the list to keep it.
Delete all files in /boot for kernels except the current and two newest kernel images. Example:
sudo rm -rf /boot/*-3.13.0-{23,45,49}-*.
Then, clean up what's making apt grumpy about a partial install :
sudo apt-get -f install
sudo apt-get autoremove
If you run into an error that includes a line like "Internal Error: Could not find image (/boot/vmlinuz-3.2.0-56-generic)", then run the command sudo apt-get purge linux-image-3.2.0-56-generic (with your appropriate version).
EvanK wrote a good python script here to automate this cleaning : https://github.com/EvanK/ubuntu-purge-kernels