LinuxBSDos.com posted a guide about upgrading Ubuntu 12.04 server to 14.04

The official recommendation for upgrading Ubuntu distributions, especially production servers, is to wait until a point version has been released. In this case, that means waiting until Ubuntu 14.04.1 was released. It was released on July 22 (2014).

So after making sure that the current system was fully dated, which meant running sudo apt-get update && apt-get upgrade, I made a snapshot of the droplet (in case the upgrade failed). Once that was done, I ran sudo do-release-upgrade. That script is part of update-manager-core, which should be installed by default.

  Upgrade Ubuntu 12.04 server to 14.04