Open the breaker box and shut off the breaker, take off the panel (usually two big screws), then pull on the breaker in the same direction as you pulled the switch to turn it off, it should rotate out then gently pull it toward you to release it. Now you will have a wire connected to it, disconnect it and replace the breaker using the same brand and amperage as the old one. Reconnect the wire push the breaker onto the bar then rotate it back into the outer connection and gently snap it into place. Turn the breaker back on, then replace the door panel. You should be good to go. Just remember dont touch anything inside the box while the door is off and make sure you are not standing on a damp or wet floor while doing this otherwise you might be getting flowers sent to your funeral. Be careful and this is an easy repair. Good Luck!!, Be SAFE!!!
Been there done that.