how to restart iphone 6s when touch screen doesnt work?

If screen not responding to touch is permanent issue after you restart you might not be able to do anything anyway.

This said, you can Force Restart your iPhone:

Force restart iPhone

https://support.apple.com/guide/iphone/force-restart-iphone-iph8903c3ee6/ios

Try a forced restart.

• Press and quickly release the volume up button

• Press and quickly release the volume down button

• Press and hold the side button.

• When the Apple logo appears, release the button.

https://support.apple.com/en-in/guide/iphone/iph8903c3ee6/ios