how I can change security questions

The page that you posted from tells you how to reset them i.e. if you have a rescue email address (which is not the same thing as an alternate email address) set up on your account then steps 1 to 5 on that page should let you reset them.

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 won't get the reset option - 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