Q: I need to change my Apple ID on my iCloud. I changed my Apple ID but it would not let me change my iCloud one.  How do I change it?

I have a new email address and want to change my Apple ID which I did manage to do but it won't let me change my iCloud email address.  How do I do this because every time I open up my phone or ipad I get a message asking me to log in to iCloud.  Had to change my email address for Apple ID back to old one to stop this happening.  Need to get them changed as no longer using old email.

Thank you for any advice.