How do I factory reset without a computer?

Go to Settings > General > Reset. If you're asked to enter your passcode and you've forgotten it, see Reset the passcode. If you're asked to enter your Apple ID password and you've forgotten it, see the Recover your Apple ID website. Tap Erase All Content and Settings.

This will also remove your personal information from your device and any settings

1. If you paired an Apple Watch with your iPhone, unpair your Apple Watch.
2. Back up your device.
3. Sign out of iCloud and the iTunes & App Store. 

• • If you're using iOS 10.3 or later, tap Settings > [your name]. Scroll down and tap Sign Out. Enter your Apple ID password and tap Turn Off.
  • If you're using iOS 10.2 or earlier, tap Settings > iCloud > Sign Out. Tap Sign Out again, then tap Delete from My [device] and enter your Apple ID password. Then go to Settings > iTunes & App Store > Apple ID > Sign Out.

1. Go back to Settings and tap General > Reset > Erase All Content and Settings. If you turned on Find My [device], you might need to enter your Apple ID and password. Forgot your Apple ID or your Apple ID password?
2. If asked for your device passcode or Restrictions passcode, enter it. Then tap Erase [device].

Settings > General > Reset > Erase All Content and Settings

How to erase your iPhone, iPad, or iPod touch – Apple Support