Can you get your battery health back up and if so how

Batteries are consumables; they lose a little capacity every time they are discharged, then recharged. On average this works out to about a 1% loss for every 25 “full charge cycles” ( some will be a little more, others a little less). As one example, if you charge the phone overnight, every night (and that is what you should do; it is a best practice), it starts the day at 100%. If it drops to 20% by the end of the day before you charge it again overnight that counts as 0.8 full charge cycles (20% to 100%), or about 24 full charge cycles per month of use. For this example your battery capacity will lose about 1% per month. Of course, if the end-of-day level is higher than 20% the capacity loss will be a little less, and if it is lower than 20%, or you charge it during the day, the capacity loss will be higher.

The absolute best way to improve the life of your battery long term is to enable Optimized Battery Charging (Settings/Battery/Battery Health) and charge the device overnight, every night. The battery will fast charge to 80%, then pause. During the night the phone will use mains power instead of battery power, allowing the battery to “rest”, and thus reducing the need to charge the battery quite as often. The phone will resume charging to reach 100% when you are ready to use your phone; it will “learn” your usage pattern.

Batteries deteriorate over time and I'm afraid there is not much you can do to restore it (other than replace it). n There are a few things you may be able to do prolong the battery life. See the 'Battery Health' section (and others) in:

iPhone Battery and Performance - Apple Support

By replacing it only.

Batteries lose capacity in time due to mechanical and chemical stress. Only fix is to replace it.