Locate the breaker you want to replace. Each breaker in the electrical box is for a different circuit within the building. Remove the panel cover with your screwdriver.
Turn off the main circuit breaker as a safety precaution when working in the electrical box. This will prevent shocks and damage to other circuits.
Use the end of the slotted screwdriver to pry the Zinsco circuit breaker away from the hot bar. The end connected to the hot bar is now free, but the breaker remains attached by the clip on the plastic bar.
Unsnap the breaker from the plastic bar. Once the breaker is free, unscrew the hot wire of the circuit from the terminal in the side of the breaker. Dispose of the old breaker.
Insert the hot wire in the hole in the side of the Zinsco circuit breaker. Tighten the screw firmly.
Attach the clip to the plastic bar, and then press firmly on the other end of the breaker to seat it in place for connection to the hot bar.
Replace the panel cover. If all work on the circuit is complete, then turn the power back on by flipping the main circuit breaker.