Apple's 6s serial number can be replaced for free.

If the serial number is within a certain range, the battery can be replaced for the equipment with the production date between September 20 15 and June 0 10. Whether its serial number is eligible for free replacement can be inquired at Apple official website. (official website meets the query)

Apple officials have confirmed that very few iPhone 6s devices may be turned off unexpectedly (battery factor). This is not a safety issue, but will only affect the equipment whose serial number is within a certain range and the production date is between September 20 15 and June 0/kloc-0.

Extended data:

Unexpected shutdown event of iPhone 6s:

Apple said that they found that "in a few iPhone 6s devices produced between September 20 15 and June 0 10, a battery element was exposed to the controlled ambient air for too long before being put into the battery pack, which led to the battery power consumption being faster than the normal level, leading to an unexpected shutdown." Apple stressed in its official statement that this is not a security issue.

Apple also mentioned in the survey results that the iPhone is designed to automatically shut down the device under certain conditions, such as extremely cold temperatures. Some cases of automatic shutdown may be unexpected to customers, but this design is to protect electronic components from low voltage damage.

References:

Apple official website -iPhone 6s Unexpected Shutdown Problem Plan

Apple official website -iPhone battery and power supply maintenance