Clean ubuntu

Clean /boot

Manual method

Kernel version

Get your current kernel version

uname -r

You should see something like:

3.13.0-32-generic

Installed kernels

Get a list of installed kernels

dpkg --list 'linux-image*'

You should see something like:

/	Name	Version	Architecture	Description
un	linux-image	<none>	<none>	(no description available)
un	linux-image-3.0	<none>	<none>	(no description available)
rc	linux-image-3.13.0-24-generic	3.13.0-24.47	amd64	Linux kernel image for version 3.13.0 on 64 bit x86 SMP
ii	linux-image-3.13.0-30-generic	3.13.0-30.55	amd64	Linux kernel image for version 3.13.0 on 64 bit x86 SMP
ii	linux-image-3.13.0-32-generic	3.13.0-32.57	amd64	Linux kernel image for version 3.13.0 on 64 bit x86 SMP
rc	linux-image-extra-3.13.0-24-generic	3.13.0-24.47	amd64	Linux kernel extra modules for version 3.13.0 on 64 bit x86 SMP
ii	linux-image-extra-3.13.0-30-generic	3.13.0-30.55	amd64	Linux kernel extra modules for version 3.13.0 on 64 bit x86 SMP
ii	linux-image-extra-3.13.0-32-generic	3.13.0-32.57	amd64	Linux kernel extra modules for version 3.13.0 on 64 bit x86 SMP
ii	linux-image-generic	3.13.0.32.38	amd64	Generic Linux kernel image

Delete kernels

Delete the kernels you don't want/need anymore

apt-get remove --purge linux-image-VERSION linux-image-extra-VERSION
apt-get autoremove

Replace VERSION with the version of the kernel you want to remove.

Example:

apt-get remove --purge linux-image-3.13.0-24-generic linux-image-extra-3.13.0-24-generic  linux-image-3.13.0-30-generic linux-image-extra-3.13.0-30-generic
apt-get autoremove

Update kernels list

update-grub