To upgrade Ubuntu to a new release usualy in a terminal just do :
sudo do-release-upgrade
If you have the error below :
Can not upgrade
An upgrade from ‘disco’ to ‘focal’ is not supported with this tool.
You must upgrade to the intermediary release first. 19.04 (disco) -> 19.10 (eoan) -> 20.04 (focal)
Edit the /etc/apt/source.list file with only this line :
deb http://fr.archive.ubuntu.com/ubuntu/ eoan main restricted
Then update you distribution with :
apt update
apt dist-upgrade
reboot
Now you can do the do-release-upgrade
Note : If you must upgrade to a release no more available in the current repository you can change the url to http://old-releases.ubuntu.com/ubuntu/
Source : Upgrading from End-of-Life Ubuntu Releases (18.10 to 20.04)