Upgrading to Ubuntu 20, *how* to back up?

[Bo Berglund]

I have seen this disclaimer on so many pages describing the upgrade to
a new Ubuntu version:

For example on this page:
https://ubuntu.com/tutorials/tutorial-upgrading-ubuntu-desktop#1-before-you-start

they write:

"We recommend that you backup your existing Ubuntu installation before
you update your computer."

But nowhere do they mention *HOW* to do the backup!

It drives me nuts when they all assume that the general user knows how
to back up his entire system.
Why can you not at least put a link on the page targeting a webpage
describing how to accomplish the backup?

As it is now I believe the majority (like me) do not know how to go
about this and just skip the backup step.

So please:
Where is a sensible easy-to-follow description on *how* to back up
prior to a system upgrade?


-- 
Bo Berglund
Developer in Sweden