How do you change 100 amp breaker in existing box?

To change a 100 amp breaker in an existing box, first ensure your safety by turning off the main power supply to the panel. Remove the panel cover and carefully disconnect the wires from the old breaker, noting their positions. Install the new breaker by connecting the wires to it in the same configuration, then securely snap the breaker back into the panel. Finally, replace the panel cover and turn the main power back on, checking that the new breaker functions correctly.