Changes in software such as upgrades may introduce new features which may add to power drain and heat generation. Check Settings > Battery feature to see if there are apps or services running that use a lot of power. If there are, decide if you need them running all the time, can turn off a service when an app is in the background, or disable the feature completely. Read this support article to learn about the battery health feature. —> iPhone Battery and Performance - Apple Support
Referring to the "Your battery's maximum capacity" section of the support article: iPhone Battery and Performance - Apple Support
"A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions." In other words, if maximum capacity is below 80%, the battery is below normal performance and worth replacing. You can still use a device with a lower health percentage than 80% but for optimal usage you may wish to consider having it changed when it reaches 80% battery health.
Go to this web site for information about how to request battery service for your iPhone: iPhone Battery Repair & Replacement - Apple Support
For cost and availability, enter your model information in the "Get an estimate" box at the bottom. (I will let you do this since the answer will depend upon the country where you are located.)
For instructions on how to get your iPhone, iPad, or iPod touch ready for service click here --> Get your iPhone, iPad, or iPod touch ready for service - Apple Support
