You can reset an iPhone, even without the password, by putting it into "recovery mode" or erasing it via iCloud's Find My ...

Recovery mode allows you to update or restore your iPhone using a computer with iTunes or Finder. Here's how to access it.

Resetting your iPhone to factory settings is an essential process for various situations, such as troubleshooting persistent issues, preparing the device for resale, or passing it on to someone else.

This video shows you the things to do before selling of giving away your iPhone. It's exciting when you get your next iPhone, but you need to be sure to properly reset iPhone to sell by making sure ...