Updating Ubuntu Linux via command line is very easy. It only takes two steps.
Step 1: apt-get update
Via SSH, type in the following to make sure your VPS / Dedicated server has the latest packages:
Step 2: apt-get upgrade
Now type in the following to upgrade:
If you are prompted whether or not you want to continue, simply type in Y and then hit Enter.
If the above steps do not work, make sure you are a sudo user or logged in as root. Try typing in the same commands as above except with sudo in front of each command (e.g. sudo apt-get update / sudo apt-get upgrade).