The upgrade needs a total of 81.5 M free space on disk '/boot'. Please free at least an additional 17.4 M of disk space on '/boot'. Empty your trash and remove temporary packages of former installations using 'sudo apt-get clean'.
http://askubuntu.com/questions/142926/cant-upgrade-due-to-low-disk-space-on-boot
Run below command as suggested in the error message.
$ sudo apt-get clean
First check your current kernel version.
$ uname -a
Then run the following command. This command will list all packages that you no longer need.
$ dpkg -l 'linux-*' | sed '/^ii/!d;/'"$(uname -r | sed "s/\(.*\)-\([^0-9]\+\)/\1/")"'/d;s/^[^ ]* [^ ]* \([^ ]*\).*/\1/;/[0-9]/!d'
So for every package listed do the following:
$ sudo apt-get -y purge some-kernel-package
Remove packages that were installed as a dependency and are no longer required using the below command.
$ sudo apt-get autoremove