Hi there
Please try this
1. Try a Deep Charge Force-Restart
Sometimes ancient batteries need a specific power kick to get past the boot loop.
- Use a high-power charger: Do not plug the phone into a computer USB port. Use a higher wattage iPad or fast-charging wall block instead.
- Leave it for an hour: Let it loop on that high-power charger for 30–60 minutes.
- Force a hard reset: While it is still plugged into the wall, press and hold the Home button and the Power button at the exact same time. Keep holding them down for at least 20 seconds. If you are lucky, this will force the phone to bypass the power-cycle error and boot to the lock screen.
2. Force "Update" via Recovery Mode
If Finder cannot see the phone, you can try forcing it into Recovery Mode. This tells the phone's hardware to talk directly to your computer before trying to fully boot the iOS system.
- Connect the iPhone 6s to your Mac/PC using a high-quality USB cable.
- Open Finder (on Mac) or iTunes (on Windows).
- Press and hold the Home button and the Power button simultaneously. Keep holding them even when you see the Apple logo.
- Release the buttons only when you see a support screen showing a cable pointing toward a computer.
- A pop-up will appear on your computer screen. Crucial: Click Update, NOT Restore.