iPhone 11 - Battery health

You cannot increase battery health. Batteries are consumable objects and the battery health will go down as they age. Apple recommends replacing the battery when it gets to 80% battery health. You should use the Get Support link to contact Apple Support about a battery replacement, or you can make a Genius Bar appointment to do it. See this, iPhone Battery Replacement - Official Apple Support

Do you experience poor battery life from the iPhone? Battery health is just a meter of the battery condition and its fine for it to drop.

Then you can get it replaced by Apple or an authorized service provider.. See the link ChrisJ4203 posted for details.