Comment 43 for bug 1573408

If you don't find the updates in Software Updater, it is because of the phased updates, which means that not everyone will get the updates at the same time to avoid mass regressions for all users. More info about phased updates is here https://wiki.ubuntu.com/PhasedUpdates. However, you will receive the updates eventually a little later.

If you want to override phased updates and update everything immediately, do this in terminal:

sudo apt update
sudo apt full-upgrade

However, that is _not_ recommended. Better rely on Software Updater.