Q: Dear, Your apple id was signed in icloud on alexanders iphone. If you believe an unauthorized person accessed your account

If the message actually started with the words "Dear, Your" it wasn't from Apple. Even if it didn't start with "Dear" it is possible that it was a phishing attempt to get you to use a link in the notice to cause you to enter your password by making it look like you were at the Apple site to change the password.

If you didn't find the Apple site to change your password on your own you may want to see the following information on how to change the password and then change the password again: Change your Apple ID password - Apple Support

If you think your Apple ID has been compromised I would follow the steps in this article. Better to be safe than sorry.

If you think your Apple ID has been compromised - Apple Support