Can I trade My iPhone 16 Pro for a iPhone 16 Pro Max?

No. The return period was 14 days. That model is not eligible for trade in at this time and most likely won’t be until late next year when new models typically come out.

Unfortunately, Apple is not accepting iPhone 16s for trade in.

Apple Trade In - Apple

If you decide to sell your current iPhone on your own, please make sure you do the following first.

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support