Install update manager

Firstly run

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sudo apt-get install update-manager

This will install the manager that will take care of updating the system.

If upgrading from a lts (long term support) version to a non lts version edit /etc/update-manager/release-upgrades and the value of Prompt= from lts to normal first.

Do upgrade

The start the upgrade process by executing

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sudo do-release-upgrade

It will then ask you if you want to proceed. Pressing y it then opens a non standard ssh port (1022 in mycase) if updating remotely to which you can connect if there is an issue and potentially fix the system.