Q: Why won't my ICloud ID change to match my Apple ID?  How can I change it?

If you have changed your Apple ID you need to sign out of iCloud on all devices connected to it, then sign back in with the new ID.

Apple ID email address or password - What to do after you change

If this is a used device, you will need to contact the seller to have them log out of iCloud or go to iCloud.com/Find My Phone and erase the device. If you can’t do that, the device is useless.