Apps store email

Hello all,

First post, ages lurking, but I can't seem to find this problem anywhere?

When I first got my 3g iphone all was good. I donwloaded some apps from the store no problem. Now when I try and the password pop up appears I have someone else's email address there.

1) why has this happened? and
2) how do I change it??

Can anyone help?

Thanks in advance,