Move the switch on the main breaker to the "Off" position. Extract the screws that secure the cover on the circuit panel with a screwdriver and lift the cover off of the box. Grip the circuit breaker that is being changed and unplug it from the box by pulling it toward you.
Loosen the screw on the breaker and pull the black wire out of the circuit breaker. Push the end of the black wire into the new circuit breaker. Tighten the screw to secure the wire to the breaker. Clip the new breaker back into the slot from which you removed the old breaker.
Lift the cover of the panel into place and reinstall the screws. Turn on the main breaker and the new breaker. The circuit should be back in service.