See -> If you think your Apple ID has been compromised
If you think your Apple ID has been compromised - Apple Support
Never jailbroken your iPhone.
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If other people have access to your iPhone or other Apple device, change the device passcode and never share it with others.
Change your Passcode -> Use a passcode with your iPhone, iPad, or iPod touch - 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
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B T W:
You are sign in here . . . with your Apple ID & Password.
Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.
You can’t merge multiple Apple ID accounts.
What happens when you use the same Apple ID & Password that you use to sign in here?
See the following Apple articles
Sign in with your Apple ID -> Sign in with your Apple ID - Apple Support
Manage and use your Apple ID -> Manage and use your Apple ID – Apple Support (UK)