总的步骤

• 查询当前内核版本: uname -a
• 列出所有内核: dpkg –get-selections | grep linux-image
• 删除老的内核: sudo apt-get remove linux-image-xxxx

Example

flying-bird@flyingbird:~$ dpkg --get-selections | grep linux-image
linux-image-3.13.0-33-generic           deinstall
linux-image-3.13.0-34-generic           deinstall
linux-image-3.13.0-35-generic           deinstall
linux-image-3.13.0-36-generic           deinstall
linux-image-3.13.0-37-generic           deinstall
linux-image-3.13.0-39-generic           deinstall
linux-image-3.13.0-40-generic           deinstall
linux-image-3.13.0-43-generic           deinstall
linux-image-3.13.0-44-generic           deinstall
linux-image-3.13.0-46-generic           deinstall
linux-image-3.13.0-48-generic           deinstall
linux-image-3.13.0-49-generic           deinstall
linux-image-3.13.0-53-generic           deinstall
linux-image-3.13.0-55-generic           deinstall
linux-image-3.13.0-57-generic           deinstall
linux-image-3.13.0-58-generic           deinstall
linux-image-3.13.0-85-generic           deinstall
linux-image-3.13.0-86-generic           deinstall
linux-image-3.13.0-87-generic           deinstall
linux-image-3.13.0-93-generic           install
linux-image-3.2.0-24-generic-pae        deinstall
linux-image-3.2.0-67-generic-pae        install
linux-image-4.4.0-34-generic            deinstall
linux-image-extra-3.13.0-33-generic     deinstall
linux-image-extra-3.13.0-34-generic     deinstall
linux-image-extra-3.13.0-35-generic     deinstall
linux-image-extra-3.13.0-36-generic     deinstall
linux-image-extra-3.13.0-37-generic     deinstall
linux-image-extra-3.13.0-39-generic     deinstall
linux-image-extra-3.13.0-40-generic     deinstall
linux-image-extra-3.13.0-43-generic     deinstall
linux-image-extra-3.13.0-44-generic     deinstall
linux-image-extra-3.13.0-46-generic     deinstall
linux-image-extra-3.13.0-48-generic     deinstall
linux-image-extra-3.13.0-49-generic     deinstall
linux-image-extra-3.13.0-53-generic     deinstall
linux-image-extra-3.13.0-55-generic     deinstall
linux-image-extra-3.13.0-57-generic     deinstall
linux-image-extra-3.13.0-58-generic     deinstall
linux-image-extra-3.13.0-85-generic     deinstall
linux-image-extra-3.13.0-86-generic     deinstall
linux-image-extra-3.13.0-87-generic     deinstall
linux-image-extra-3.13.0-93-generic     install
linux-image-extra-4.4.0-34-generic      deinstall
flying-bird@flyingbird:~$ uname -a
Linux flyingbird 3.13.0-93-generic #140-Ubuntu SMP Mon Jul 18 21:20:08 UTC 2016 i686 i686 i686 GNU/Linux
flying-bird@flyingbird:~$ sudo apt-get remove linux-image-3.2.0-67-generic-pae 
[sudo] password for flying-bird: 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
  account-plugin-identica account-plugin-twitter fcitx-libs-gclient gir1.2-ebook-1.2 gir1.2-ebookcontacts-1.2 gir1.2-edataserver-1.2
  gir1.2-messagingmenu-1.0 gir1.2-networkmanager-1.0 libllvm3.8 libmircommon5 snap-confine ubuntu-core-launcher
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
  linux-image-3.2.0-67-generic-pae
0 upgraded,0 newly installed,1 to remove and 3 not upgraded.
After this operation,114 MB disk space will be freed.
Do you want to continue? [Y/n] y
(Reading database ... 320329 files and directories currently installed.)
Removing linux-image-3.2.0-67-generic-pae (3.2.0-67.101) ...
Examining /etc/kernel/postrm.d .
run-parts: executing /etc/kernel/postrm.d/initramfs-tools 3.2.0-67-generic-pae /boot/vmlinuz-3.2.0-67-generic-pae
update-initramfs: Deleting /boot/initrd.img-3.2.0-67-generic-pae
run-parts: executing /etc/kernel/postrm.d/zz-update-grub 3.2.0-67-generic-pae /boot/vmlinuz-3.2.0-67-generic-pae
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-3.13.0-93-generic
Found initrd image: /boot/initrd.img-3.13.0-93-generic
Found memtest86+ image: /boot/memtest86+.elf
Found memtest86+ image: /boot/memtest86+.bin
Found Ubuntu 12.04 LTS (12.04) on /dev/sda7
done
flying-bird@flyingbird:~$

这里的12.04 LTS是当初升级Ubuntu时备份的老的操作系统。