Since August 1. 2014 Skype 4.2 doesn't work anymore. If you try to connect you get the error "Skype can't connect". To update Ubuntu 12.04 or Ubuntu 14.04 you can proceed as follows:
1. Install From Skype's Website
You can download the latest version of Skype as a .deb package directly from its website. Choose the 12.04 (multiarch) version if you're running Ubuntu 12.04 LTS or newer; it'll still work if you're on i.e. 14.04 LTS, because that's just what Microsoft lists as the minimum Ubuntu version.
Once you have it, run the following commands to ensure your system is clear of any previous installations:
sudo apt-get remove skype skype-bin:i386 skype:i386
Then double-click on the downloaded .deb file to open it in the Ubuntu Software Center and install it like any other software.
Afterwards, also run the following command in the terminal to ensure that support for the indicator menu is available:
sudo apt-get install sni-qt:i386
Now you can find Skype in the dash, run it and enjoy it!
2. Install From Ubuntu Software Center
As of August 2, 2014, you can now install Skype 4.3 directly from the Ubuntu Software Center.
To do this, you must first enable the Canonical Partner Repository, using the following steps (based on steps provided by OMG! Ubuntu; see source):
Then search for Skype in the Ubuntu Software Center and install it as you would any other software.