How can I force unattended-upgrades to run on demand?

You can execute unattended-upgrade with -d option to run it any time you want :)

Open terminal and execute this command:

sudo unattended-upgrade -d

It will start checking for available updates and if found it will upgrade your system.

In addition to this I also want you to go to this site:

https://help.ubuntu.com/18.04/serverguide/automatic-updates.html

^{May 24, 2018: Changing the URL for18.04 as 12.04 is expired.}

Reply if you need any clarification.

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