i have attached a generic external webcam to my laptop running elementary os freya but i can't see to find a way to change which webcam it uses, it always displays the builtin webcam. Thanks.
This currenty is not possible, but there already is a bug report filed on Launchpad. You could sign in here and press the "This bug affects me too" button, so the problem get's more attention.
You could try Cheese in the meantime, which does support multiple webcams. You can install it by typing this in the terminal:
sudo apt-get install cheese
Or you could install it with the softwarecenter.