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/?caller=kbase&SG=SG003&category_id=SC0105&symptom_i d=23306
Be advised to book with the genius bar of your local store as there are bound to be many others with the same need. In addition to that it has become known that 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 world wide and will last continue at least December 2018.