How do I remove an Apple ID from an Apple device?

See these support articles:

Restore your iPhone, iPad, or iPod to factory settings [with a computer] - "A factory restore erases the information and settings on your iPhone, iPad, or iPod and installs the latest version of iOS or iPod software."

Click here --> Restore your iPhone, iPad, or iPod to factory settings - Apple Support

How to erase your iPhone, iPad, or iPod touch

"Learn how to wipe information from your iPhone, iPad, or iPod touch and restore your device to factory settings." Without using a computer. Does not reinstall iOS.

Click here --> How to factory reset your iPhone, iPad, or iPod touch - Apple Support

If neither of the above work, read the "If you can't erase your device" section in the last link, including the last part about contacting Apple.

Note that you may need to sign out of an old account first. If you do not have that information Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. Do this online at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not detail what currently constitutes 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). This will give you access to the device but won't restore access to the account used to lock it. There is nothing that can be done to expedite an Activation Lock request.

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