ここの手順そのまま。
<a href="https://wiki.ubuntu.com/FocalFossa/ReleaseNotes#Upgrading_from_Ubuntu_18.04_LTS_or_19.10" target="_blank" rel="nofollow noopener noreferrer">FocalFossa/ReleaseNotes - Upgrading from Ubuntu 18.04 LTS or 19.10 - Ubuntu Wiki</a>
<blockquote>
To upgrade on a server system:
<ol>
<li>Install update-manager-core if it is not already installed.</li>
<li>Make sure the Prompt line in /etc/update-manager/release-upgrades is set to 'normal' if you are using 19.10, or 'lts' if you are using 18.04 LTS.</li>
<li>Launch the upgrade tool with the command sudo do-release-upgrade on 19.10; use sudo do-release-upgrade -d if you are using 18.04 LTS.</li>
<li>Follow the on-screen instructions.</li>
</ol>
</blockquote>

Ubuntu 18.04 LTS を 20.04 LTS へアップデートする

Discussion