iCloud password change
I can’t access my iCloud email. How do I change my password? It’s.not the same as my Apple ID.
iPhone 6, iOS 12
If you can't access mail the account with web mail address it can be added manually in phone Add an email account to your iPhone, iPad, or iPod touch - Apple Support
The Apple ID is different other than mail account address , when Apple ID is signed in the device the Apple ID password can be changed Change your Apple ID password - Apple Support
The password for web mail account can't be changed when mail account is already added from iPhone , however it can be changed from the web account , say the web mail address is example1@gmail.com see the link https://support.google.com/mail/answer/41078?hl=en&ref_topic=2467017