Q: Just retired and need to change my iPhone iCloud ID to my new iCloud account. How do I change it?

Just retired and need to change my iPhone iCloud ID from my company email to my new iCloud account that uses my personal email address. How do I change it? Also, I tried to log into iCloud using my old company email ID but the password no longer works, so couldn't see if there was some way inside that to change the ID.