You can use these instructions to hard reset your iPhone:

For models of iPhone 8 or later:

Toggle the Volume Up button on and off.
Toggle the Volume Down button on and off.
Once the Apple logo appears on the screen, press and hold the Side button, also referred to as the Power button. Even after the power-off slider displays, hold down the Side button.
When you see the Apple logo, release the Side button.

Regarding iPhones 7 and 7 Plus:

Simultaneously press and hold the Volume Down and Side (or Sleep/Wake) buttons.
Hold down both buttons until the screen displays the Apple logo.
As soon as you see the Apple logo, release both buttons.