iphone 6s plus will not connect to itunes.

Welcome to the Apple Support Communities!

I understand you are trying to connect to the iTunes application on your Windows computer. If iTunes is not recognizing your device, try disconnecting it and follow the steps below:
If iTunes doesn't recognize your iPhone, iPad, or iPod
If you are trying to restore your device are unable to do so on your computer, you can complete the steps below on any computer with iTunes software.
If you can't update or restore your iPhone, iPad, or iPod touch​

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 with these steps, but don't release the buttons when you see the Apple logo, wait until the connect to iTunes screen appears:

   On an iPhone 6s and earlier, iPad, or iPod touch: Press and hold both the Home and the Top (or Side) buttons at the same time. Keep holding them until you see the connect to iTunes 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 the connect to iTunes screen, let the download finish, then repeat step 3.
   
   
5. After the Update or Restore completes, set up your device.

Get more help

If you can’t update or restore your device with recovery mode, or you can't use recovery mode because of broken buttons, contact Apple Support.

Take care.