how can I change security questions?

Welcome to the Apple Community.

You might try to see if you can change your security questions. Start here, change your country if necessary and go to manage your account > Password and Security.

I'm able to do this, others say they need to input answers to their current security questions in order to make changes, I'm inclined to think its worth a try, you don't have anything to lose.

If that doesn't help you might try contacting Apple through Express Lane (select your country, navigate to iCloud help and enter the serial number of one of your devices)