But what if you only want to upgrade certain individual packages?
apt-get upgrade will upgrade ALL or nothing. So, that is out of the question.
What you need is apt-get install. apt-get install serves dual purposes: install and upgrade. It installs a package if it is not already installed. If it is already installed, apt-get install will upgrade the package to the latest version.
You should still run apt-get update before apt-get install. You can specify one or more package names as arguments to the apt-get install command.
For example, to upgrade these 2 packages: libfreetype6, libtiff4:
$ sudo apt-get update
$ sudo apt-get install libfreetype6 libtiff4