Check out is page for more information
https://www.apple.com/iphone-battery-and-performance/
There is no fault in the batteries of these phones, Apple released a software feature meant to protect and extend the longevity of these phones life and battery life by reducing the strain of use (slowing down certain strenuous activities)
Apple is allowing battery replacements at a reduced cost of $29 plus tax on iPhones:
6, 6 Plus, 6s, 6s Plus, 7, and 7 Plus.
Until December 2018.
The best way to get a battery replaced is to use the Apple Support App (download here )
https://itunes.apple.com/app/id1130498044?mt=8
Or call Support 1 (800) 275-2273
Have them run a diagnostic and tell you the capacity of your battery currently so you can decide if you want to change it now or wait until you actually need it. If you don’t needn’t it they will still change it for you until December 2018. ORDER THE BATTERY WITH SUPPORT because all stores are experiencing low inventory of all batteries.
If they order a battery for you you just have to wait for an email from the store you choose stating they have the battery and you can come drop off your phone for the repair ( which could take anywhere from 1-4 hours depending on the # of phones they already have to repair)
**be sure to create an iCloud or iTunes backup before going in because they cannot do this for you and it could take time to complete ( call support if you need help)