There are two ways to factory reset an iPhone:

1. Using Settings:

   1. Go to Settings > General > Transfer or Reset [Device].
   2. Tap Erase All Content and Settings.
   3. Enter your passcode or Apple ID password if prompted.
   4. Tap Erase to confirm.

2. Using Recovery Mode:

   1. Connect your iPhone to a computer using a Lightning cable.
   2. Force restart your iPhone:
      • On iPhone 8 or later: Press and quickly release the Volume Up button, then press and quickly release the Volume Down button. Finally, press and hold the Side button until you see the recovery mode screen.
      • On iPhone 7 or earlier: Press and hold the Side button and the Volume Down button at the same time until you see the recovery mode screen.
   3. On your computer, open iTunes (or Finder on macOS Catalina or later).
   4. When you see the option to Restore or Update, choose Restore.
   5. iTunes or Finder will download the software for your iPhone. If the download takes more than 15 minutes and your iPhone turns off, force restart it again.
   6. Once the software is downloaded, iTunes or Finder will restore your iPhone.

Once your iPhone is factory reset, it will be erased of all data and settings. You will need to set it up again as if it were a new device.