How to upgrade to the latest Emacs?

You can install the nightly packages from the ubuntu-elisp ppa.

sudo add-apt-repository ppa:ubuntu-elisp/ppa
sudo apt-get update
sudo apt-get install emacs-snapshot

Emacs 24.4 is still relatively new. Ubuntu is considered a stable distro, and for that reason software sometimes lags behind. If Emacs 24.4 isn't packaged, you'll need to build it yourself.

You can find comprehensive instructions on how to install GNU Emacs 24.4 on Ubuntu in this guide: http://ubuntuhandbook.org/index.php/2014/10/emacs-24-4-released-install-in-ubuntu-14-04/