How do I factory reset my device?

I have looked everywhere for an answer of how to factory reset my tablet. It used to belong to someone in my family and they forgot the password. Sure, lots of sites have ways to do it from being off, but instead of what they say happening, it always gives me this message:

Warning!!

-----------------

A custom OS can cause critical

problems in phone and installed

applications.

If you want to download a custom

OS, press the volume down key.

Otherwise, press the volume up

key to cancel.

-----------------

Volume up : Cancel (restart phone)

Volume down : Continue

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And this is before it even gets to the Samsung logo.