how can i restart my iphone

See the following from Restart your iPhone - Apple Support

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

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