iOS 10 - iPhone 6s plus, upgrade fail iTunes

Hi lpsandaruwan,
I understand that you are getting errors when trying to update your iPhone to iOS 10. I know it's important to have up to date software on your devices, and I'm happy to help.
Check out this article for help with issues while updating your iPhone:
If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support

Put your device in recovery mode and set it up again

1. Make sure that you're using the latest version of iTunes.
2. If iTunes is already open, close it. Connect your device to your computer and open iTunes. If you don't have a computer, borrow one from a friend or go to an Apple Retail Store or Apple Authorized Service Provider for help.
3. While your device is connected, force restart it: Press and hold both the Sleep/Wake and Home button for at least 10 seconds, and don't release when you see the Apple logo. Keep holding until you see the recovery mode screen.
   
4. When you see the option to Restore or Update, choose Update. iTunes will try to reinstall iOS without erasing your data. Wait while iTunes downloads the software for your device.
   If the download takes more than 15 minutes and your device exits recovery mode, you'll need to repeat these steps and choose Restore instead of Update when you get back to this step.
   
5. After the Update or Restore completes, set up your device. If you previously backed up your device with iTunes or iCloud and had to restore your device in step 3, you might be able to restore from your backup during setup.

If you continue to have problems with those steps, this article addresses this error code specifically:
If you see Error 3194, Error 17, or 'This device isn't eligible for the requested build' in iTunes - Apple Support
Thanks for using the Apple Support Communities. Cheers!