How to restart my iphone 6

How to restart your iPhone SE (2nd or 3rd generation), 8, 7, or 6

1. Press and hold the side button until the power-off slider appears.
2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button until you see the Apple logo.

Restart your iPhone - Apple Support

Try a Forced Restart, noting the specific instructions for the iPhone 6.

Force restart iPhone - Apple Support

Settings>General>Shutdown, then hold the power button on the right side until it restarts.