My iPhone 12 pro max, iOS 14.6 Battery health drop so fast. Please help me for increase battery health.

The question asked above is good to know. But you should also understand Battery Health does NOT increase. Batteries are consumable parts and with every full cycle charge, Battery Health declines little by little. Battery Health will continue to decline over time and when the Health goes to 80% or less, it will then be time to have the battery replaced in your phone. This is what batteries do. Over time they lose their ability to hold the same charge as when new. It's normal and expected behavior.

There are things you can do maximize the performance of the battery in your phone, such as turning on Optimized Battery Charging and plugging your phone in to charge at night while you sleep, leaving it in to charge all night long, every night. Also, never let the battery become fully depleted or dip below 20% charge. See this link --> Batteries - Maximizing Performance - Apple

What percentage is it at and how long have you owned the phone?