To get the best use out of your system, it's best to have your system up to date. I'll quickly go over the differences and then we have the various how to's in the sections below : This type of install is where you move to a more recent revision of the operating system, whilst keeping all of your programs, settings and data intact.
Upgrading to or Installing the latest version of Skype in Ubuntu is a simple as downloading the right package, opening it, and hitting has finished downloading, open it by double-clicking, and you'll see a Software Centre Screen that has an Upgrade Button in the top-right-hand-corner.
Each upgrade looks to add new features and make old features work better.
It also looks to make itself more compatible with new hardware and software.
Occasionally, the old version of Skype may refuse to be replaced, in which case Uninstall it from the Ubuntu Software Centre (search, don't open the .deb) then proceed with the installation as described above.
This document describes how to upgrade from Ubuntu 12.04 to Ubuntu 14.04 LTS.