14.04 - Software updater needs more disk space

I got the following error message from Software Updater:

The upgrade needs a total of 74,6 M free space on disk '/boot'. Please free at least an additional 68,3 M of disk space on '/boot'. Empty your trash and remove temporary packages of former installations using 'sudo apt-get clean'.

How can I get more disc space? I have

df -h
Filesystem                    Size  Used Avail Use% Mounted on
/dev/mapper/xubuntu--vg-root  454G   13G  418G   3% /
none                          4,0K     0  4,0K   0% /sys/fs/cgroup
udev                          1,9G  4,0K  1,9G   1% /dev
tmpfs                         384M  1,3M  383M   1% /run
none                          5,0M     0  5,0M   0% /run/lock
none                          1,9G   24M  1,9G   2% /run/shm
none                          100M   36K  100M   1% /run/user
/dev/sda2                     237M  219M  6,1M  98% /boot
/dev/sda1                     511M  3,4M  508M   1% /boot/efi

I experienced this same message with Ubuntu 14.04. The answer to this question took care of the problem by removing old Linux kernels from /boot. It freed almost a half gig of space there.

Here's the link:
How do I remove old kernel versions to clean up the boot menu?