Hello.
- Go Settings.
- Tap Backup & reset and then Reset device.
- Tap Erase everything
If you cant access the phone's settings then follow the instructions below.
- Turn off your Galaxy S6
- Press and hold together the Volume Up + Home + Power button.
- Release all buttons when you see Samsung logo.
- Then select "wipe data / factory reset" using Volume buttons to navigate and Power button to confirm.
- Choose "Yes -- delete all user data".
- Select "reboot system now". when reset is complete.