Installing Security Updates in Ubuntu

To begin installing security updates, the first thing is to update your repository index, which would instruct the server to check all lists of packages that are new or out-dated.

Update your repo with the following command:

sudo apt update

Once, you’ve done that, you will need to upgrade them if any new packages are available.

You can either run

sudo apt upgrade

or 

sudo apt dist-upgrade

The difference here is that apt upgrade will automatically install an update without removing any packages while the apt dist-upgrade will update new packages, install new packages as a dependency of the upgraded packages, and remove packages that are broken by upgraded packages.