how to remove old kernels in grub2, including files?

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.