Safely remove old files in /boot

First you need to know which kernel you are running. You do that with

# Check the current kernel version
uname -a
# List the old kernel
sudo dpkg --list 'linux-image*'|awk '{ if ($1=="ii") print $2}'|grep -v `uname -r`
# Where linux-image-XXX should accordingly be filled in with the kernel version you want to delete
sudo apt-get remove --purge linux-image-3.11.0-XX-generic
# Remove old packages
sudo apt-get autoremove
# Update grub kernel list
sudo update-grub