If you have a rescue email address (which is not the same thing as an alternate email address) set up on your account then the steps half-way down this page should let you reset them : http://support.apple.com/kb/HT5312
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 HT5312 link above to add a rescue email address for potential future use
The Three Best Alternatives for Security Questions and Rescue Mail
1. Use Apple's Express Lane.
Go to https://expresslane.apple.com ; click 'See all products and services' at the
bottom of the page. In the next page click 'More Products and Services, then
'Apple ID'. In the next page select 'Other Apple ID Topics' then 'Forgotten Apple
ID security questions' and click 'Continue'. Please be patient waiting for the return
phone call. It will come in time depending on how heavily the servers are being hit.
2. Call Apple Support in your country: Customer Service: Contact Apple support.
A substitute for using the security questions is to use 2-step verification: