How to remove old kernels in Grub2, including files.
Using the Disk Usage Analyzer, I found that the Linux kernels are located in 4 areas of the root directory: /boot /lib/firmware /lib/modules /usr/src
All we have to do is, go to each directory and remove the files or directories that are not needed.
Use the command [rm] for files and [rm -r] for directories. You have to use [sudo su] to have administrative account, or use [sudo (command)] to invoke administrative command.
For our example, we will use the linux kernel version 4.4.0-112 You can choose which version you will remove.
at /boot = kernels are all files : abi-4.4.0-112-generic config-4.4.0-112-generic initrd.img-4.4.0-112-generic retpoline-4.4.0-112-generic System.map-4.4.0-112-generic vmliniz-4.4.0-112-generic
(note: not all kernel versions have these files included above)
at /lib/firmware = kernels are in subdirectories : 4.4.0-112-generic
at /lib/modules = kernels are in subdirectories : 4.4.0-112-generic
at /usr/src = kernels are in subdirectories : linux-headers-4.4.0-112 linux-headers-4.4.0-112-generic
Now all we need to do is use the command [update-grub]
This will free your hard disk around 300MB worth of disk space for every kernel that you will remove.
Please note that I am using Ubuntu 16.04 Xenial.
And if anybody knows how to delete files in GUI, please let me know, it would be alot easier.