How to upgrade my iPhone 13 to 16

You are referring to the Apple Trade In Program. You can choose to have this done in-store at an Apple Store or Online and ship it in. If you do this in-store a specialist will evaluate the condition of your device and offer you a trade in value, you can then apply it to your new iPhone's purchase in store. If you do this by mail you can select your new iPhone online then you have 14 days after receiving it to mail in your old iPhone for evaluation and trade in to apply the credit.

Learn more here and find out an estimated value of your current device: iPhone Exchange Offer - Apple Trade In - Apple

You can go into Apple's online store and go to the iPhone 16's "Buy" page. Under the iPhone image, there's the Trade-In section. Click on "Select Smartphone" and answer the questions posed to get a value estimate for your existing device.

click here ➜ Buy iPhone 16 and iPhone 16 Plus - Apple

Apple has a Trade In program, which you can read about here --> Apple Trade In - Apple