Read this message from Apple regarding an exchange program for iPhone batteries for iPhone 6 and upwards. Your phone should qualify.
https://support.apple.com/iphone/repair/battery-power
For instructions about what to do, visit this link. https://getsupport.apple.com/?
If you go to your local Apple store instead, be sure to make a reservation; there will be many with similar thoughts. In addition it has become known that battery stock is low and many countries are waiting supplies. Because of the enormity of this operation, people will need to be patient.
The offer is available world wide and will continue until at least December 2018.