I'd previously been running Xubuntu 16.04 on my main laptop without any issues, and had been waiting for a while before upgrading to Xubuntu 18.04.
Because I was having problems with bluetooth, and some furious googling had lead me to conclude that a distribution upgrade would resolve my issues, I decided that now was a suitable time to update my operating system.
Xubuntu's documentation states that when there is a major LTS release available for you to upgrade to, the update manager will pop up with a dialogue box informing you, and giving you the option to update. It looks similar to this:
You will not get this prompt if you have broken PPA repository URLs configured. The best way to find out which PPA URLs are causing problems, is to run...
$ sudo apt-get update
From your terminal, and observing the results:
Reading package lists... Done
E: The repository 'http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial Release' does not have a Release file
In my case, my PPA configuration for the excellent image editor, Pinta, appeared to be broken. Simply disabling this PPA from the software updater by unchecking the checkbox allowed the OS to fully pull update information, and then prompted me with the distribution update dialogue.