Trade in

There's different ways depending upon the age of the phone and how you want to do it.

If you bought the phone less than 2 weeks ago you can return it to an Apple store.

You can sell the phone and buy a new one. You can give it back to Apple for a credit but usually selling will get you more.

iPhone Trade-Up program - https://www.apple.com/iphone/trade-up/ - "Just trade in your eligible smartphone, and then use the credit to lower the full cost of a new iPhone or to reduce the monthly payments with your carrier." Not available in all countries.