Follow this guide to replace the battery in an iPhone 6s Plus.
Use this guide to bring life back to your iPhone 6s Plus with a new battery. For optimal performance, calibrate your newly installed battery: Drain battery below 10%, then charge uninterrupted to 100%
This guide instructs you to detach the front panel assembly; this is intended to prevent damage to the display cables. If you feel comfortable supporting the display carefully while peeling the battery out of the iPhone, you can skip the display removal and go directly to the battery removal steps.