Important: Take a server snapshot at your provider before upgrading.
Once you've taken a snapshot of your server, SSH in to your server as root and run the following commands to make sure your server is ready to be upgraded to a new version of Ubuntu.
sudo apt-get update && sudo apt-get dist-upgrade sudo dpkg --configure -a
If any of the above commands fail or print error messages, do not proceed with upgrading to a newer Ubuntu version.
Next, run the following command to remove any conflicting apache2 packages in case you ever accidentally installed them while customizing your server:
sudo apt-get remove apache2 apache2-bin apache2-data
To upgrade your Ubuntu 12.04 server to Ubuntu 14.04, SSH in to your server as root and run the following command:
sudo do-release-upgrade -f DistUpgradeViewNonInteractive
If the upgrade completes without errors, take a new snapshot of your server so you'll have a snapshot of the server after the upgrade.
If the upgrade fails or encounters any errors, restore your server to the snapshot you made before you began the upgrade.