Well, it's in /etc/apt, and it's called sources.list, but that's obviously not clear to you, so let's try this...
On a command line,
cd /etc/apt
ls -la
at that point will show you:
*
sources.list, which is a file that contains various sources, and
*
sources.list.d, which is a directory that contains files that contain more sources
Most likely in sources.list
You need to edit it as root, so I'd use @sudo vi <file>@. You may prefer something like @sudo nano <file>@, though, or even @sudo mousepad <file>@. It depends what GUI editors you have installed.
If you're entirely GUI-based, and want to avoid the command line, start the Ubuntu Software Centre - select Edit -> Software Sources, and that's doing the same in a more user-friendly format.
You can also get at it through the Synaptic Package Manager. There, the same editing window is under Settings -> Repositories.
Take your pick :)