Switch to 16

If you purchased your phone from Apple, you have 14 days to return it for a full refund. You can then, pre-order the iPhone 16 starting this Friday. That would be the correct way to proceed. There is no exchange option where you would just pay the difference.

You have 14 days from the date of purchase to return any undamaged item, no questions asked.

Simply return the iPhone 15 for a full refund and purchase an iPhone 16 when it's released.

If you did not buy directly from Apple, any exchange options are solely up to the seller, not Apple.