how to retrieve or change my iCloud email address password?

Try -- If you forgot your Apple ID password - Apple Support.

I had this same question because the iCloud email set up by itself without me entering a password, and it added itself along side my gmail messages in the "Mail" app on my iPad, without entering a password. Afterwards, I checked iCloud.com, and I was able to see my iCloud mail after entering my Apple ID and password. So, I guess they figure that if you can open your iPad, then you can open your mail without a password. On my Mac it was also added along side my gmail messages, again without entering a password, but I had to click the button at the top left of the screen, "get new messages" before the iCloud messages would show up. So, I don't think there is a separate iCloud email address password, but only the Apple ID and password. This simply explains what I have experienced with this issue. Hope this helps