How to find out if your Apple iPhone 6s is eligible for a free battery replacement
Now, Apple has revamped the page on its website to include a "serial number checker." Type in your iPhone 6s' serial number and you will quickly learn if your phone is among those possibly affected by this issue, which could cause the battery to suddenly fail. The serial number of an iPhone can be found by going to Settings > General > About.
If your iPhone 6s was produced during the time period that suggests a problem with the battery, you need to back up your data to iCloud or iTunes, disable Find My iPhone, and erase all data and settings. This is accomplished by going to Settings > General > Reset > Erase all Content and Settings. Once these things are done, you can bring the device to an authorized Apple repair shop, or an Apple Store.
Phones eligible for a free battery replacement must be in good working condition. Those with a screen cracked so badly that it interferes with the battery replacement, might have to replace the screen first before getting the battery exchanged. According to Apple, in some circumstances there could be a cost associated with the swap.
If you paid to have the battery on your iPhone 6s replaced, you can request a refund from Apple. The iPhone 6s free battery replacement program runs until the third anniversary of the date of the handset's first sale. That means that it should run into September 25th 2018.
To get to the "serial number checker" for your iPhone 6s, click on the sourcelink.
source: AppleSupport via MacRumors