How can I change my answer to security question

If you can remember their current answers then you can login here and change them : https://appleid.apple.com

If you can't remember their answers and you have a rescue email address (which is not the same thing as an alternate email address) set up on your account then you can try going to https://appleid.apple.com/ and click 'Manage your Apple ID' on the right-hand side of that page and log into your account. Then click on 'Password and Security' on the left-hand side of that page and on the right-hand side you might see an option to send security question reset info to your rescue email address.

If you don't have a rescue email address then see if the instructions on this user tip helps : https://discussions.apple.com/docs/DOC-4551