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
Recognize and avoid phishing messages, phony support calls, and other scams
Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support
Phony "tech support" / "ransomware" popups and web pages
Phony "tech support" / "ransomware" popup… - Apple Community
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To keep your Apple ID account secure, see the following Apple articles:
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
If other people have access to the 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
Never jailbroken the iPhone.
