The first and most important thing to understand is: It is NEVER NEVER NEVER, that the breaker is too small for the water heater.
If the electric water heater was working fine, then the breaker keeps tripping, the worse thing you can do is install a larger circuit breaker. you will cause a fire and burn your house down.
Ok now what is causing the breaker to trip?
A leak inside the water heater may be dripping on to the wires. this can be very dangerous make sure the power is off before you remove the element covers, and remember MOST BREAKERS ARE MARKED WRONG, do not trust that they are marked correctly this can cost you your life.
A bad circuit breaker.
Sometimes a breaker goes bad and needs to be replaced, but it must be replaced with the same size, smaller or larger will cause problems.
Most common cause is a bad thermostat inside the water heater. once again make sure the power is off.
If you are going to replace the thermostats you may as well go ahead and replace the elements at the same time.
The thermostats and the elements are all that control an electric water heater and the total cost of both thermostats and both elements is not much at all.
Please keep in mind different water heaters use different elements get the information from your water heater before you go and buy your thermostats and elements.
It's not just the wattage of the elements but there are different thread sizes and some are bolt on and others are screw in.
All the information you need should be written on the side of the water heater.