Change Apple ID on phone

With no background it is hard to provide a simple, clear answer.

The simplest answer is to sign out of the old ID and sign into the new one.

Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac - Apple Support

At the other extreme you could be needing this:

'Activation Lock for iPhone, iPad, and iPod touch' is a "feature that's designed to prevent anyone else from using your iPhone, iPad, or iPod touch, if it's ever lost or stolen." To learn more click here --> Activation Lock for iPhone, iPad, and iPod touch - Apple Support

In most cases to remove an activation lock absolutely requires the information for the account that applied the lock. Click on this link to learn 'How to remove Activation Lock' --> How to remove Activation Lock - Apple Support

If you do not know the required information and you are not the original owner, get the owner of the account to remove the lock or return the device for a refund or recycle the device. The device is otherwise useless to you. If the owner is unknown and has not provided contact information as a message showing in the login screen, there is no way to contact the owner unless the person who gave you the device knows them.

Here are additional resources if you are the original owner or can find the original owner:

Help with a lost Apple ID or password:

• "If you forgot your Apple ID" [this is likely an email address you are using or used in the past] click here --> If you forgot your Apple ID email address or phone number - Apple Support

• "If you forgot your Apple ID password" click here --> If you forgot your Apple ID password - Apple Support

Reset device:

Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. This is done online at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not provide details as to what is acceptable proof. Prior to this new online service Apple typically required the original receipt* issued by Apple or an Apple Authorized Reseller and documented provenance (so typically it would be the original owner who does this, or a descendant with legal proof of ownership).

*Apple recommends you "Keep your receipt after you buy an Apple device" --> Keep your receipt after you buy an Apple device - Apple Support