It will remove all the data ,contacts and apps that you installed
First power off your phone.
Then press and hold together 3 keys: Volume Up + Home button + Power button.
When you see logo Samsung, then release all holded keys.
Next select option "wipe data/factory reset" -> use Volume buttons to browse, and Power button to confirm option.
Now choose "yes" with Volume Down, and confirm with Power button.
After that press Power button, to confirm option: "reboot".
Turn on you cell phone.
After that browse to Settings -> User and Backup -> Factory Data Reset.
Next please confirm info on your screen.
Switch on device.
Dial or press number to call: *2767*3855#
That number code will force format on device.
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Restore Hard reset will set your phone in origional factorry settings. It will remove all the data and apps that you installed. Your contacts will be removed and you have to add them again.