How do I select which server Ubuntu will get it's software and updates from? I know how to find the list of mirrors on the web, but not how to use them.
Open the Software & Updates (you can also start this using the
Find the Download From dropdown and select Other
Choose the server you want:
If you click Select Best Server, the USC will go through a load of tests to find the server that's fastest in your location. Generally it will be a server close by.
For a lot more information on this, see the Ubuntu Community Documentation on Repositories