From here.

After the “20.04” version is released, a dialog window may be popped up in your “Ubuntu 18.04” desktop to ask you whether you want to upgrde to “20.04”. If you close it, you can still use the following commands to do the same thing:

Run the following commands:

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# Upgrade all the packages which is required to do the upgrading.
sudo apt-get update
sudo apt-get dist-upgrade
# Rebooting may be needed here.
# Remove the useless pacakges.
sudo apt autoremove
# Do the version upgrading.
sudo do-release-upgrade