Upgrade Ubuntu 18.04 to 20.04 LTS

Do Upgrade:

sudo apt update && sudo apt upgrade
sudo reboot
sudo apt install update-manager-core
sudo do-release-upgrade
sudo reboot

Caution:
update and upgrade was note running well on my machine, and found that "TeamViewer" was not be updated for some reason.
do:

sudo apt purge teamviewer

to uninstall teamviwer. (Later reinstall at Ubuntu 20.04 LTS)