Replacing 10 amp breaker for 20 amp
The short answer is that the breaker protects the wire (otherwise, heat and fire can result). You can put a 20A breaker on a circuit if all the wire on the circuit is #12. If any of the wire is #14 you can put a 15A breaker on it. If any of the wire in the circuit is smaller than #14, then you cannot put a 15A breaker on it.