How to clean the /boot directory on LINUX Mint?

After yesterday's update of Linux Mint Mate (64bit), I started to receive a warning about "Full disk space". I have 400 MB for the /boot partition on my hard disk.

This is the screen-shot of the /boot files.

This is the screen-shot of the update-log.

The question is: How can I clean the /boot? What files are safe to delete?

I received this response from StackOverflow before they flag my question...

Check the current kernel version with uname -r and make sur that the current kernel work without any problem to keep it.

From the terminal:

List the installed kernel :

dpkg --list | grep linux-image
dpkg --list | grep linux-headers

Remove the old kernels through the following command:

apt-get purge linux-image-x...
apt-get purge linux-headers-x...

using the synaptic package manager

Open Synaptic and mark the old kernel versions for removal then apply changes

Update Manager

from the Update Manager view the Linux kernels , click the 'Remove' button for the kernels that you want to remove

Purge-old-kernels

You can install the purge-old-kernels cli tool through:

sudo apt-get install bikeshed

If you want to purge the old kernel except for the latest 2 kernels, run the following command:

sudo purge-old-kernels --keep 2

Finally run

sudo update-grub 
sudo update-grub2

Get the current of list of installed kernel packages:

dpkg -l linux-* | awk '/^ii/{ print $2 }' | grep -e [0-9]

View current kernel version:

uname -r

Take away the old versions of packages that are older than the current kernel by specifying them separated by a space in command:

sudo apt-get purge