I want to install a tankless water heater. Have a 125 amp panel, with two 30 amp breakers for the current water heater. Install manual recommends having two 40 amp breakers. Max draw is 75 amps. Can I just replace the two 30 amp breakers with two 40 amp breakers?
Not likely will it be possible to replace the two 30A breakers with 40A breakers.
The wire size (AWG) for a 30A circuit should be 10 gauge. Proper wire size for a 40A circuit is 8 gauge if the run from the service entrance to the tankless heater is a reasonably short distance.
So do not simply replace the breakers like you asked. The breaker's primary function is to protect the wiring from over current so as to prevent a fire in overload or short circuit conditions.