How Do You Replace The Thermostat In An Electric Hot Water Heater?

There are many varieties of hw heaters.  First, shut off the power even if you are just going to look it over.

There should be a removable access door with one or two screws.  If it is a large heater there are probably two panels, one higher and one lower.  After you remove the door, you will see insulation and directly behind the insulation are the 220 volt wires attached to the thermostat (don't even think about reaching thru the insulation with the power on).  Remove the wires and remove the thermostat.
How will you know which thermostat is bad--upper or lower?  Are you sure it is the therm. And not an element that is burned out?
You may have to do some testing.  I will say that I have replaced a number of elements but never had a thermostat go bad, that I can recall anyhow.
If I didnt have the means to test the existing thermostats, I would go ahead and drain the tank and remove and inspect the elements.  A bad element is usually easy to determine.  Don't turn on power til the tank is totally refilled as it can destroy the elements.
Turn off the power, please.  A HW heater with water and good grounds, not to mention 220 volts, presents ideal conditions for electrocution.

Top thermostat keeps popping to off ?

If you don't know the answer to that ... Just don't mess with it. .. Call an electrician or plumber.