upgrade

You can trade in with Apple. See this article (also the FAQs at the bottom): Apple Trade In - Apple

or, you can try selling it by yourself, then add the money needed to buy a new model.

Prices can be found here (obviously that depends on what model and what capacity you're interested in:--> iPhone - Apple

Regards

Giulio

Start here: https://www.apple.com/shop/trade-in