Upgrade Ubuntu Release

#!/bin/bash
set -e

# https://linuxconfig.org/how-to-upgrade-to-ubuntu-18-04-lts-bionic-beaver

sudo apt update  -y
sudo apt upgrade -y
sudo apt dist-upgrade -y

sudo apt autoremove -y

sudo apt install update-manager-core -y

sudo reboot

sudo do-release-upgrade

# lxc exec NAME -- script /dev/null -c do-release-upgrade

# container:~# script /dev/null
# do-release-upgrade

a2enmod php7.2

You may need to update postgres and other apps

sudo pg_dropcluster 10 main --stop
sudo pg_upgradecluster 9.5 main
sudo pg_dropcluster 9.5 main
apt purge postgresql-9.5

sudo do-release-upgrade -d will force upgrade from Ubuntu 20.04 LTS to Ubuntu 20.04 LTS. In case you receive an "Upgrades to the development release are only available from the latest supported release." message, make sure that release upgrader default behavior is set to ltswithin /etc/update-manager/release-upgrades

ref

• https://linuxconfig.org/how-to-upgrade-ubuntu-to-20-04-lts-focal-fossa
• https://help.ubuntu.com/lts/serverguide/installing-upgrading.html