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 and you may want to get the help of a professional
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.