Step 1. Tap Settings, then General. Scroll down until you see Reset.
Step 2. Tap Erase All Content and Settings.
Step 3. Just in case you tapped the button by accident, your iPhone or iPad will ask you to confirm the reset, and when you tap OK it will start the process.
Please Note: This will erase all data & applications from your device! Please ensure you have copied all required data before carrying out this irreversible procedure.