App Store :: How To Change AppleID Security Questions And Answers

I recently got a letter from apple that my apple ID birthdate/security questions & answers had been changed. I think this was because of a previous letter that told me I had to update my account with the new security features, but I don't recall. 

Anyway, I decided to check on the appleID site, got logged in, changed my password successfully, but I couldn't see how to change the security questions or answers - and the answers I did try submitting weren't accepted. 

So now I'm stuck: my account is OK, my password is OK, but I can't remember the security answers and can't see how to change them. 

Info:
imac intel, Mac OS X (10.6.7)