The short answer is to add the next apt repository to the Third-Party Software (or Other Software in newer versions) in Software Sources (or Software & Updates in newer versions):
deb http://old-releases.ubuntu.com/ubuntu code_name main restricted universe multiverse
The long answer...
Well, actually we will do this without using any terminal. Not even once. Just GUI, I promise ;-)
First, open Software Sources (or Software & Updates in newer versions). It does not matter how old your Ubuntu is, there is certainly something like this. For Ubuntu 9.04 (Jaunty Jackalope), look at next image to see where it's located:
After Software Sources (or Software & Updates) is open, go in Ubuntu Software and Updates tabs and unselect everything like in next pictures. You don't need these things anymore since your Ubuntu version is End of Life:
Without closing Software Sources (or Software & Updates), go in Third-Party Software (for newest releases, this tab is named Other Software) tab and add a new apt repository. Insert exactly next line when you are asked:
deb http://old-releases.ubuntu.com/ubuntu jaunty main restricted universe multiverse
If your version of Ubuntu is not 9.04, replace in the above line
jaunty with your Ubuntu codename (for example, if you have Ubuntu 9.10, replace it with
karmic and so on):
Now, when you will close Software Sources (or Software & Updates), you will be asked to reload the information about available software. Just be sure that you have a working internet connection:
And now, you are free to download almost whatever you want. For 9.04, you can use Synaptic Package Manager. For newest releases, there is Ubuntu Software Center.
For example, to install VLC in Ubuntu 9.04 using Synaptic Package Manager, follow the instructions in the following pictures:
If you want to Update your Ubuntu to a new release, just go to System > Update Manager:
I tested this method from a live session of Ubuntu 9.04 (Jaunty Jackalope) and as you can see from these pictures it worked. If you are on an installed session of Ubuntu, you will be asked sometimes for root or admin password. Just insert your personal user password when you are asked.