Switching from UK to US store - how?

Friends,

While in the UK, I tried to download a new app and it wouldn't let me because my credit card is from the US, but now that I've returned to the US, I'm unable to update my apps - the screen says "Account Not In This Store... Your account is not valid for use in the UK store. You must switch to the US store before purchasing." How do I make this switch?

With thanks in advance,

Frances Sheehan