How to extend the warranty of iphone 12 pro max?

How to buy AppleCare+ in your Settings App

*If your iPhone is still eligible

1. On your iPhone, iPad, or iPod touch, open Settings.
2. Go to General > About.
3. Tap AppleCare+ Coverage Available. If you don't see AppleCare+ Coverage Available, check to make sure that your device is eligible.
4. Follow the onscreen instructions to complete your purchase. You can see the price of the AppleCare plan when you follow these steps.

https://support.apple.com/en-us/HT202702