iPhone data erasing help
1. Tap the settings (gear icon) from the home screen (or whatever page it happens to be located on your iPhone).
2. Tap the "General" settings menu item.
3. Choose the "Reset" menu item.
4. Tap on the "Erase All Content and Settings" menu item.
But this won't completely erase your data from iPhone, and the data can be easily recovered by some data recovery tool.
So, I suggest you try Safewiper iPhone data eraser to wipe your iPhone clean without recovery before selling.
Safewiper is a professional data wipe software which can help you erase everything from iPhone.