My security question answers don't work, how do I change them?

I have 4 computers synced to my iTunes account. It asks me security questions to which I know the answers, but every time I put them in, they are incorrect. I obviously have 2 computers that work perfectly to download, purchase, make CD's etc. But when I try to do that with the other 2 computers, it asks me for the security question answers as "THIS IS THE FIRST PURCHASE WITH THIS COMPUTER" and then I can't get the answers. How can I go about removing them? Or just changing them? It's obviously my account but this is just irritating. How many FIRST CARS CAN I HAVE!?!?!