The 3D Touch feature on iPhone 6S (Plus) brings some new functionality when touching the screen of your iOS device. So, to update all apps on iPhone 6S at once using 3D Touch feature, you just need to do as follows:
1. Find the App Store icon On the home screen of your iPhone 6S (6S Plus)
2. There should be a red badge on the app's icon, indicating the number of apps requiring update
3. To bring the 3D touch options, press the App Store icon lightly to open the menu which includes items like: Search, Redeem and Update All
4. Simply touch the Update All option and all your apps will start to update immediately.
Note that, you can also open the App Store, go to the Updates tab, and tap the Update All button on the top right of the screen to update all your iPhone 6S apps at once.