my iPhone has been stolen

This document has a list of things to do if your iPhone, iPad, or iPod touch is lost or stolen. --> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

If you can access your Apple ID, you can change security questions through these steps from Change your Apple ID security questions - Apple Support :

"1. Sign in to appleid.apple.com.

2. In the Sign-In and Security section, select Account Security.

3. Under Security questions, select Change security questions.

4. Choose your new security questions and their answers, then select Update.

5. Enter your Apple ID password, then select Done.

It's very important to remember the answers to your security questions to avoid being locked out of your account. And make sure that you add and verify your rescue email address ( About your Apple ID email addresses - Apple Support ). If you forget the answers to your security questions, you'll need a rescue email address to help you reset them."

If you forgot the answers to your security questions, try the directions in the archived (no longer updated) article If you forgot the answers to your Apple ID security questions - Apple Support for resetting your security questions:

"Reset your security questions

1. Go to iforgot.apple.com.

2. Enter your Apple ID, then select Continue.

3. Choose the option to reset your security questions, then select Continue.

4. Enter your Apple ID password, then select Continue.

5. Follow the onscreen steps to verify your identity. The information you’re asked to provide may vary based on your account details and other factors. If you don't get any options to verify your identity, you can't reset your security questions at this time.

6. Choose new security questions and answers and select Continue. See our tips for choosing security questions and keeping your account secure ( Security and your Apple ID - Apple Support )."

If you get a message about not sufficient information then realize that several years ago Apple transitioned to using to two factor authentication instead of security questions. Apple has been encouraging users to switch to the newer security system, two factor authentication. Your next recourse will be try to change to two factor authentication as outlined in this article --> Two-factor authentication for Apple ID - Apple Support Follow the instructions in the section "Turn on two-factor authentication for your Apple ID"

If you still have problems then contact Apple for help with Apple ID account security using the information in this document --> Contact Apple for help with Apple ID account security - Apple Support

Additional reference:

- Post by Community Specialist How do I fix/reset answers to the two sec… - Apple Community