How can I change my phone to 12 pro max

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.

If you bought your phone from Apple in the past 2 weeks you can return it for a refund and get a different one.