Q: Friend gave me a AirPort Extreme 2011

You will need to perform a factory reset. With most models this involves unplugging the power lead, holding the small reset button down with the tip of a ball-pen or pencil, then plugging the power back in - it sounds as if you may need three hands for that.

This page gives detailed instructions for various models, including pictures showing where the reset button is.

Resetting an AirPort base station FAQ - Apple Support