How do you change your rescue email if you have forgotten your security questions?
You won't be able to change the rescue email address until you can answer 2 of your questions, you will need to contact iTunes Support or Apple to get them reset.
e.g. you can try contacting iTunes Support : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Account Management , and then 'Forgotten Apple ID security questions'
or try ringing Apple in your country and ask to talk to the Accounts Security Team : http://support.apple.com/kb/HE57
When they've been reset you can then use the steps half-way down this page to update your rescue email address for potential future use : http://support.apple.com/kb/HT5312 . Or you could change to 2-step verification : http://support.apple.com/kb/HT5570