Monthly payment

You may be asking about two separate services.

To get more information about the Apple Trade In service click on the link --> Apple Trade In - Apple

then

iPhone Upgrade Program - Apple

Kristenk17 wrote:

Can I trade in my iPhone 12 at apple and get an unlocked 14 pro max

Yes.

As far as the monthly payment is concerned you need to contact the Apple Store. iPhone Payments Terms & Conditions