If you forgot the Apple ID (iCloud) password, enter the Apple (iCloud) ID, and click Forgot Password. See if you can get a Rescue or Notification email option. Link to contact Apple Support at the end of the 2nd article.
Apple ID - Manage
Apple ID – Forgot Password *
If you can recover the password, 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 *