how do i change facebook security question ?

[Saqib]

I don't think there is any security question option on Facebook. But you can change your yahoo security question to avoid being fully hacked and choose a question, which you think, cannot be answered by just anyone, like something nobody, especially the hacker, can guess. Just block the hacker from your Facebook account and don't let him know you user name or email id.
Put privacy on, in case the hacker uses another account. Finally, have a secondary email address for you Facebook account. I think these are more than just enough.