Locate the female ring of the peephole. The female ring is the portion of the peephole that unscrews and is usually located on side of the door that closes, facing inside the room or house. Underneath, where the female ring meets the door, there are two slots on either side for flat head screwdrivers.
Insert the flat head screwdrivers into either slot, and rotate both counterclockwise to unscrew the female ring from the door. Keep the ring safe in a pocket or in a hand until it can be reattached.
Straddle the door at the edge. Push the peephole from the inside, out to the other side. Catch the peephole tube from the outside with your free hand.