This post describes the update of xUbuntu 18.04 LTS to xUbuntu 20.04 LTS (focal). The upgrade process has been launched with

sudo do-release-upgrade -d

and everything went smoothly except for two things:
• First, chromium now seems to be available only through snap. It does not require to do anything special but it is something to remember.
• Second, xscreensaver needs to be deactivated for the upgrade. I did it and have not fixed this issue since (so the session is just locked with the standard xfce tool and displays a black screen).

After the upgrade, these minor fixes have been performed:

• A few repositories have been deactivated for the update, including **R** and nextcloud client. I reactivated them by (for **R**) adding the line
deb https://cloud.r-project.org/bin/linux/ubuntu focal-cran40

to the file /etc/apt/sources.list and (for nextcloud) by editing the file /etc/apt/sources.list.d/nextcloud-devs-ubuntu-client-DIST.list to uncomment the line
deb http://ppa.launchpad.net/nextcloud-devs/client/ubuntu focal main

Then,
sudo apt update

sudo dpkg -i gcstar_1.7.1+repack-1_all.deb