You need to fully pay off the iPhone 11 Pro Max before you can trade it in for the iPhone 12 Pro Max. Open the Wallet app, select your Apple Card, select the 3 dots in the upper right, select Monthly Installments, scroll down an select Pay Early. Just know that the iPhone 11 Pro Max only has a $500 trade in value. Just for the base model iPhone 12 Pro Max you're looking at $1099. So if you want to do everything at once you pay $1419 for the iPhone 11 Pro Max and then $599 with your trade in for the iPhone 12 Pro Max so you'll need just over $2000 + tax. If you're wanting to trade in every year you may be better off signing up for the Apple Upgrade Program.
iPhone Upgrade Program - Apple
You also could probably get a lot more value for the iPhone 11 Pro Max if you sold it yourself rather than trading it in. You're basically paying $1419 to get a $500 credit.