How to fix the battery health on my iPhone XR?

What do you mean by how to fix battery health? Battery Health is a measurement of the batteries ability to hold the same charge it did when new. With EVERY full cycle battery charge, the health of ALL batteries declines. There is no way to fix this as it's a natural chemical aging of a rechargeable battery. Once the health of a battery in an iPhone dips to 80% or less, it's useful life has been spent and it needs to be replaced by Apple or an Apple Authorized Service Provider.