Oh i see, well before you do anything, do:
apt-get update && apt-get upgrade
So you know what you're doing, I am going to explain what that command does. It is two commands joined together with a '&&'.
The two commands by them selves are:
You could run those commands in that order separately and get the same result as the one command i've given you (assuming everything goes right).
apt-get is your package manager, it used to install programs. When you tell it to 'update'. It checks you /etc/apt/sources.list file. That file has all the locations/reps on where to get packages. apt-get update checks those locations to see what packages are there, so your computer knows what is available.
When you tell apt-get to upgrade, it actually downloads packages from that place, and installs/upgrades them. I hope that's clear. Either way, it's only optional reading, if you're interested in learning.