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Upgrading from Ubuntu 18.04 LTS or 19.10
You can upgrade to Ubuntu 20.04 LTS from either Ubuntu 18.04 LTS or Ubuntu 19.10.
Ensure that you have all updates installed for your current version of Ubuntu before you upgrade.
To upgrade on a desktop system:

Open the "Software & Updates" Setting in System Settings.

Select the 3rd Tab called "Updates".
Set the "Notify me of a new Ubuntu version" drop down menu to "For long-term support versions" if you are using 18.04 LTS; set it to "For any new version" if you are using 19.10.
Press Alt+F2 and type update-manager -c -d into the command box.

Update Manager should open up and tell you that Ubuntu 20.04 LTS is now available.
If not you can run  /usr/lib/ubuntu-release-upgrader/check-new-release-gtk

Click Upgrade and follow the on-screen instructions.
To upgrade on a server system:

Install update-manager-core if it is not already installed.

Make sure the Prompt line in /etc/update-manager/release-upgrades is set to 'normal' if you want non-LTS upgrades, or 'lts' if you only want LTS upgrades.

Launch the upgrade tool with the command sudo do-release-upgrade -d

Follow the on-screen instructions.
Note that the server upgrade will use GNU screen and automatically re-attach in case of dropped connection problems.

There are no offline upgrade options for Ubuntu Desktop and Ubuntu Server. Please ensure you have network connectivity to one of the official mirrors or to a locally accessible mirror and follow the instructions above.