How to change the security questions for Apple ID if forgotten?

Unfortunately it does not work that way. If that was the case, then someone who was trying to gain access to your account and did not know your security answers, could change them as well. Two Factor Authentication does away with the security questions and has been the recommended option for many years. There is an old archived support article that is no longer being updated that may still work:

If you forgot the answers to your Apple ID security questions - Apple Support

Otherwise contact Apple directly to see if they offer an updated solution:

Contact Apple for support and service - Apple Support

Several years ago Apple transitioned to using two factor authentication instead of security questions. You can continue to use security questions if you already use them and they are working for you, but if you have problems then they no longer get direct support. You can try the directions in this support article If you forgot the answers to your Apple ID security questions - Apple Support but the article is labeled "archived" and may contain information that is out of date. If the instructions work then fine; if they do not (e.g., “not enough information”) then you will have to change to the newer security method if you can. Over the past few years Apple has been persistently encouraging users to switch to the newer security system. For those who have disregarded their encouraging, Apple will likely only help you change to two factor authentication, if even that is possible. Follow the instructions in the "Turn on two-factor authentication for your Apple ID" section in this support article --> Two-factor authentication for Apple ID - Apple Support to try to change to two factor authentication.

If you still need help, contact Apple ID support at this link ➞ https://getsupport.apple.com/?caller=cups&PGF=PGF63005

Select "Other Apple ID Topics", then "Security concerns or account compromised." This will give you a chat or telephone call option.

If Apple can not or will not help you transition to two factor authentication then it is quite possible you will have to simply abandon the old account and everything it contained.

Good luck!

The “Best Answer” - and possibly the only answer - is to simply register for Apple’s 2FA if at-all possible.

Better for you both now, AND going forward into the future.

Two-factor authentication for Apple ID - Apple Support