I finally got my flying wing..........but forgot I needed a pusher prop. My question is, can I just flip a regular prop backwards to create the reverse thrust OR mount the prop like regular and reverse the polarity on the motor so it spins backwards? Thanks.

Reverse the polarity. It will work. Not as efficiently but it will work.

Just putting a prop on backwards will still provide thrust in the same directon, in fact that was a trick in the days of RC with rudder only was to install the prop backwards when the airplanes climb rate was excessive.

If you reverse the polarity reversing the motor then the prop will need to be installed backwards.

John