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 apt upgrade
    upgraded everything.
  • The software gcstar is no more available. I downloaded the version available for Ubuntu 18.04 LTS at https://launchpad.net/ubuntu/+source/gcstar/ and installed it with: 
    sudo dpkg -i gcstar_1.7.1+repack-1_all.deb
sudo apt install -f
    However, this temporary fix is still unsatisfactory and I hope that gcstar will somehow reappear in the Ubuntu official software library.