I get the below error when I try to install updates using Software Updater:

Not enough free disk space
The upgrade needs a total of 81.3 M free space on disk '/boot'.
Please free at least an additional 15.9 M of disk space on '/boot'.
Empty your trash and remove temporary packages of former installations using 'sudo apt-get clean'.

What should I do?

firmtechnologies@FirmTechnologies:~$ (ls -l /boot)
total 155801
-rw-r--r-- 1 root root  1158016 May  3 01:30 abi-3.13.0-24-generic
-rw-r--r-- 1 root root  1161713 May  8 01:31 abi-3.13.0-26-generic
-rw-r--r-- 1 root root  1161713 May 15 20:07 abi-3.13.0-27-generic
-rw-r--r-- 1 root root  1161764 Jun  4 22:57 abi-3.13.0-29-generic
-rw-r--r-- 1 root root   165510 May  3 01:30 config-3.13.0-24-generic
-rw-r--r-- 1 root root   165538 May  8 01:31 config-3.13.0-26-generic
-rw-r--r-- 1 root root   165521 May 15 20:07 config-3.13.0-27-generic
-rw-r--r-- 1 root root   165544 Jun  4 22:57 config-3.13.0-29-generic
drwxr-xr-x 5 root root     1024 Jun  6 14:31 grub
-rw-r--r-- 1 root root 29091568 May  7 21:31 initrd.img-3.13.0-24-generic
-rw-r--r-- 1 root root 29094684 May 12 12:24 initrd.img-3.13.0-26-generic
-rw-r--r-- 1 root root 29095678 May 18 10:57 initrd.img-3.13.0-27-generic
-rw-r--r-- 1 root root 29093700 Jun  6 14:32 initrd.img-3.13.0-29-generic
drwx------ 2 root root    12288 Apr 30 17:11 lost+found
-rw-r--r-- 1 root root   176500 Mar 12 13:31 memtest86+.bin
-rw-r--r-- 1 root root   178176 Mar 12 13:31 memtest86+.elf
-rw-r--r-- 1 root root   178680 Mar 12 13:31 memtest86+_multiboot.bin
-rw------- 1 root root  3372643 May  3 01:30 System.map-3.13.0-24-generic
-rw------- 1 root root  3377429 May  8 01:31 System.map-3.13.0-26-generic
-rw------- 1 root root  3377429 May 15 20:07 System.map-3.13.0-27-generic
-rw------- 1 root root  3378267 Jun  4 22:57 System.map-3.13.0-29-generic
-rw------- 1 root root  5776416 May  3 01:30 vmlinuz-3.13.0-24-generic
-rw------- 1 root root  5790912 May  8 01:30 vmlinuz-3.13.0-26-generic
-rw------- 1 root root  5790912 May 15 20:07 vmlinuz-3.13.0-27-generic
-rw------- 1 root root  5792544 Jun  4 22:57 vmlinuz-3.13.0-29-generic