The 3D Touch features on the iPhone 6s and 6s Plus brings a new dimension when touching the screen. To turn off / turn on 3D Touch on iPhone 6s (plus), do as follows:
1. Open the Settings app on your iPhone 6s
2. Go to the General settings, scroll down to the Accessibility option and Open it
3. Open the 3D Touch settings
4. Now, to enable/disable 3D Touch on iPhone 6s and iPhone 6s Plus, toggle the 3D Touch button On/Off
5. You can also change the 3D Touch Sensitivity from Light to Medium to Firm, and specify the amount of pressure required to use the 3D Touch features.