How to backup before a release upgrade from Ubuntu 18.04.5 LTS server?

[Bo Berglund]

My Ubuntu server, which is hosting subversion and a few websites, needs to be
upgraded to 20.04.3 LTS from 18.04.5 LTS18.04.5 LTS.
I have found a command line tutorial on how to do this here:
https://www.cyberciti.biz/faq/upgrade-ubuntu-18-04-to-20-04-lts-using-command-line/

It looks OK except for this:
Step 1 (important) is to make a backup, but it says NOTHING about how to do
that..

I have tried to figure out how but failed, most search hits talk about setting
up regular backup systems for the *data*, and they assume there is a desktop
too. But this is a headless server accessed via SSH using PuTTY on Windows.

What I need is something that can restore the ubuntu system itself rather than
the data.

I have a 500 GB disk drive with 199 GB free space.
It stores a lot of video content (198 GB) as well as a subversion server
repository (6 GB). And the Ubuntu system of course...

I would like to get advice on how I can make a backup from which can restore the
18.04.5 system if the upgrade fails but will not include the videos.

I have a mount to a Synology NAS on my network in fstab, which is accessed on
/nfs/backup, but it has not been used so is empty. The NAS has 2.3 TB free
space.

What should I do in order to be safe before starting the do-release-upgrade?

Note that I need the LetsEncrypt certificates to not stop working and the
certbot updates of certificates to still be in place!

-- 
Bo Berglund
Developer in Sweden