What can I do about my battery health

if your battery health has dropped below 80%, then you could get the battery replaced.

If your details are correct and you have an iPhone 6, might be a good time to consider upgrading instead, since the iPhone 6 is more or less obsolete at this time. The SE is pretty equivalent to the iPhone 6 and is the lower price model.

iPhone Battery Replacement - Official Apple Support

If you have an iPhone 6, and it has the original battery, most likely the "best" thing would be to have the battery replaced by either Apple or an Apple Authorized Service Provider.

In addition, the following articles may prove helpful:

• Maximizing Battery Life and Lifespan - Apple
• iPhone Battery and Performance - Apple Support
• iPhone Battery & Power Repair - Apple Support

If your health is below 80 percent, have the battery replaced.