See -> If you forgot the answers to your Apple ID security questions
If you forgot the answers to your Apple ID security questions - Apple Support
⤵️
Reset your security questions
- Go to iforgot.apple.com.
- Enter your Apple ID, then select Continue.
- Choose the option to reset your security questions, then select Continue.
- Enter your Apple ID password, then select Continue.
- 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.
- Choose new security questions and answers and select Continue. See our tips for choosing security questions and keeping your account secure.
After you reset your security questions, you might want to set up two-factor authentication instead. It's the best way to keep your account secure, and you won't need to remember any security questions. You can set up two-factor authentication right from your iPhone, iPad, or iPod touch with iOS 9 or later or your Mac with OS X El Capitan or later.
If you answer your security questions incorrectly too many times in a row, you temporarily can't use your security questions. Try to reset your questions after the waiting period ends.
-----
To keep your Apple ID account secure, see the following Apple articles:
If you think your Apple ID has been compromised
If you think your Apple ID has been compromised - Apple Support
See who has access to your device or accounts -> See who has access to your iPhone or iPad – Apple Support (UK)
Delete suspicious content from your devices
Delete suspicious content from your devices – Apple Support (UK)
Check your Apple ID device list to see where you're signed in
Check your Apple ID device list to find where you're signed in - Apple Support
- if you see devices that don't belong to you, delete from your account.
Personal Safety User Guide -> Personal Safety User Guide – Apple Support (UK)
Manage and use your Apple ID -> Manage and use your Apple ID – Apple Support (UK)
(if you haven't already done this) Two-factor authentication for Apple ID -> Two-factor authentication for Apple ID - Apple Support
Change your Apple ID password -> Change your Apple ID password - Apple Support
- Create a strong password for your Apple ID, It is best if your password consists of eight or more characters and include upper and lowercase letters, and at least one number. You can also add extra characters and punctuation marks to make your password even stronger.
- Never provide your password, passcode, security questions, verification codes, recovery key or any other account security details to anyone else. Apple will never ask you for this information.
Then see this Apple article: What to do after you change your Apple ID or password
What to do after you change your Apple ID or password - Apple Support
Never jailbroken your iPhone.