How do I get a refund on unused portion of AppleCare+, if I upgrade to new iPhone?

See the following for information about cancelling and refunds: How to manage your AppleCare plan - Apple Support

Hi there,

I believe that the option is non-refundable or transferable, and that the service can not exactly be “cancelled” as such.

Hope this helps.

You don't.

The warranty stays with the device.

Not true Jonathan. Apple will refund the unused portion of an AppleCare plan.