Here is what you need to do if you want to upgrade your computer to the latest version. First go to ubuntu.com, from there you will want to click on the download button on the left hand side. From there you will choose the architecture that is on your computer either x86, 64bit, or PPC, you should know what it is or find out. Then you will want to choose which version of Ubuntu you want. Currently the stable version is 7.10, however if you wait 3 more days the newest version 8.04 will become stable, and you can choose that. After you choose which version you want you can downlaod the .iso file. Then all you have to do is burn the .iso file to a CD or DVD, and you are all set.
A word of advice you need to use an ISO burning program for the .iso file. You can't use a music buring program, it won't burn correctly. Check out this thread for some options or do a google search for one: