This is a serious move as it will return the mobile to the state it was in before you set anything up on it. That means you will lose your contacts, your stored text messages, and any apps that you've downloaded to the mobile. As you start to return the mobile to the factory settings, you will see several warning messages to this effect.
- On the home screen of the Galaxy S4 or S3, select Apps
- Scroll left if needed and tap Settings
- On the S4 tap the Accounts tab at the top of the screen, then tap the Backup and reset menu item
- On the S3 tap the Back up and reset menu item
- Tap the Factory data reset menu item
- Tap the Reset device button
- Tap the Erase everything button
Your phone will now reset and will begin the initial start up process.