I want to change my iPhone 6 to iPhone 15

Then visit an Apple store and buy an iPhone 15. You can't change an iPhone to a newer model

You can go to Apple Store and the employees will be happy to help you to move all of your existing data in iPhone 6 to 15. Don’t forget to bring the payment for 15.

Apple would be happy to sell you an iPhone 15. Your iPhone 6, however, has no trade value at this point. It's simply too old. You may have a family member you can pass the phone to?

Are you by any chance saying that you have purchased an iPhone 15, and now want to change the tag line on your posts from "iPhone 6s Plus" to "iPhone 15"?!?