I am installing a new tankless water heater that calls for a 40 amp breaker. I have a 30 amp breaker available on the sub-panel that I can use, but want to replace it with 40 amp. Granted the wiring coming out of the sub-panel to the heater is up to specifications, is it as simple as unplugging the 30amp breaker and replacing it with the 40 amp one or does an electrician needs to do some work on the wiring under the sub-panel where the new breaker will plug in.

Thanks!

Are you assuming the wiring from the 30A breaker will be correct for a 40A breaker? If so then this is NOT a safe assumption.

This is not as simple as replacing a breaker.

I agree with SFM. Need to do a wire upgrade. Is the tankless unit only 120V? Most are 240V and require 2 breakers. 120V on a small tank won't give you much hot water.