Enter the Apple (iCloud) ID, and click Forgot Password. See if you can get a Rescue or Notification email option if necessary. Link to contact Apple Support at the end of the 2nd article.
Apple ID - Manage
Apple ID – Forgot Password *
You can also add 2 Factor Authentication to your account for more security. If the attempts continue, you might want to consider a name change.
Link to contact Apple Support in the article.
Apple ID has been compromised. *
Security and your Apple ID. *
Checklist: If you want to see if anyone else has access to your device or accounts *
ASC YouTube for How to reset your Apple ID password
Do you want to change your Apple ID email address on your current Apple ID so you retain access to your data and prior purchases? Note the restrictions in the article.
Apple ID Name Change. *
Make sure you turn off Find My Phone before signing out of the old ID. Once signed out and Find My Phone is off, then sign in with the new ID, then turn Find My Phone back on.
What to do after you change your Apple ID or password - Apple Support * I suggest doing this before you change your Apple ID or password.
Apple ID -Two-factor authentication *
ASC YouTube for what is two-Factor Authentication
Two Factor Authentification availability. *
2 Factor Authentication - How to generate a recovery key. *
Apple ID - Using app-specific passwords. *
Apple ID - Two Step Verifications FAQ *