To view what version of Ubuntu you currently have installed on your box, the easiest is simply to enter the falling in from the command line:

cat /etc/issue

or even

cat /etc/lsb-release

Finally, if you feel like getting your fingers a little deeper in the details surrounding your machine, you can make use of the uname command, the switches for which can all be looked up in the man pages (but -a should give you what you want!)