I have 40 gal Rheem hot Water Heater, reacently changed out both Themostats. Still water very hot, turned of power, drained tank, turned water back on, filled the tank, made sure all wires were hooked up, turned on power, waited about 1-2 hours, tried hot water. Scalding hot again. Power in again shut off. What else could be problem...

How long has this been going on? How old is the water heater?
Has there been any prior work or repairs?

Since you have the thermostat control on each element and the over temperature safety control device, I can think of no way for this to occur unless something has been wired incorrectly.

You said you have changed out both thermostats. You do mean thermostats and not elements don't you?

That heater has two elements and two t-stats. I would think it would be the one you did not change. Always test something first; don't just change parts because you will usually be wrong. DrTom4444