If you know the answers to your current questions then steps 1 to 5 on the page that you posted from should let you change them : go 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 if you answer your questions then you can chose different questions.
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 those steps should give you a reset link beneath the 2 questions.
If you don't have a rescue email address (you won't be able to add one until you can answer 2 of your questions) then you will need to contact iTunes Support / Apple to get the questions reset.
Contacting Apple about account security : http://support.apple.com/kb/HT5699
When they've been reset (and if you don't already have a rescue email address) you can then use the steps half-way down the page that you posted from to add a rescue email address for potential future use