Replacing the battery on the iPhone 4S requires minimal disassembly. [ … ] For optimal performance, calibrate your newly installed battery: Drain battery below 10%, then charge uninterrupted to 100%.

To reassemble your device, follow these instructions in reverse order. [ … ] If your iPhone reboots constantly after the replacement, you need to do a "Hard Reset" by holding down the Home- and Sleep-Button for about 10 seconds (until the device restarts). [ … ] If you have WiFi and/or Cellular connection problems with the new battery, try restoring the device using iTunes. [ … ] For optimal performance, calibrate your newly installed battery: Drain battery below 10%, then charge uninterrupted to 100%.