Each Apple device needs to be registered and/or signed in to with an Apple ID account. The name of that Apple ID account is in the form of an email address. As it is personal and a security measure, one can only change it after successful sign-in with the current Apple ID credentials. Else anyone could assign it to themselves.
If you forgot your Apple ID - Apple Support
If you forgot your Apple ID password - Apple Support
Apple ID support - Official Apple Support
If you purchased the device from someone else, or were gifted this, then the previous owner should have signed out for the device and the services on it. What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support