Upgrade Xubuntu 18.04 to 20.04

I would always recommend reading the release notes for the release you are wanting to move to, for Xubuntu the release notes are

https://wiki.xubuntu.org/releases/20.04/release-notes#updates

They however do not have Xubuntu specific release upgrade notes, they instead refer you to the Ubuntu 20.04 release notes which are

https://wiki.ubuntu.com/FocalFossa/ReleaseNotes

which includes the terminal command (see upgrading servers)

sudo do-release-upgrade -d

but I'd recommend you read the release notes first, rather than copy/paste this command into your system (starting with the Xubuntu notes first). Release notes include a list of known issues which were encountered by testers of the system providing warning of issues you may encounter.

If you're willing to wait, you will be offered the opportunity to upgrade your system (with fewer issues expected) after the upgrade path is normally opened a few days after the Xubuntu 20.04.1 official release.

apt update

is used to check and update your package repository for new versions of your installed packages.

apt upgrade

is used to "update" those packages to their new versions.

Here is a website that describes the process of manually upgrading to a new distro-release via GUI or command-line